Module: Module 1 — Variables, Data, and Arithmetic
Estimated Time: 45–60 minutes
Prerequisites: Lesson 1.1 — Variables, Lesson 1.2 — Strings

Learning Goal

By the end of this lesson, you should be able to store numbers in variables, perform arithmetic with them, calculate percentages, and predict how Python evaluates mathematical expressions.

1. The Need

So far, your programs can store and display information.

For example:

passage = "Acts 17:11"
print(passage)

But suppose you are building a Bible reading tracker.

You know:

Total chapters: 40
Chapters completed: 15

Now you want Python to answer questions:

How many chapters remain?
What percentage of the plan is complete?
How many chapters could be read each day?
Is there a remainder after dividing the chapters across several days?

Strings cannot solve those problems.

We need Python to work with actual numbers.

2. Numbers Are Different From Strings

Look at these two values:

"40"

and:

They look similar.

But Python treats them differently.

This:

"40"

is a string.

It is text.

This:

is a number.

Because there are no quotation marks, Python can use the value in arithmetic.

For example:

chapters = 40

The variable chapters now refers to a numeric value.

3. Integers

Whole numbers in Python are called integers.

Examples:

You can store integers in variables:

total_chapters = 40
completed_chapters = 15

These values are numbers, not text.

That means Python can perform arithmetic with them.

4. Floats

Numbers containing decimal points are commonly represented as floats.

Examples:

1.5
3.14
25.0
87.5

Example:

hours_studied = 2.5

The value:

2.5

is a float.

For now, the most important distinction is:

17      → integer
17.5    → float
"17"    → string

Quotation marks matter.

The decimal point matters.

5. Addition

Python uses:

for addition.

Example:

monday = 3
tuesday = 4

total = monday + tuesday

print(total)

Output:

Python retrieves the values:

and:

and adds them.

6. Subtraction

Python uses:

for subtraction.

Example:

total_chapters = 40
completed_chapters = 15

remaining = total_chapters - completed_chapters

print(remaining)

Output:

Now our program is doing something useful with the data instead of merely displaying it.

7. Multiplication

Python uses:

for multiplication.

Example:

chapters_per_day = 4
days = 7

total = chapters_per_day * days

print(total)

Output:

Do not use:

as the multiplication symbol.

Python uses:

8. Division

Python uses:

for normal division.

Example:

chapters = 30
days = 5

daily_amount = chapters / days

print(daily_amount)

Output:

6.0

Notice something interesting.

Even though:

30 ÷ 5 = 6

Python’s / operator produces:

6.0

That is a float.

For now, remember:

Normal division with / produces a floating-point result.

9. Integer Division

Python also provides:

//

This is called floor division.

For positive numbers, you can initially think of it as:

Divide and keep the whole-number portion.

Example:

chapters = 17
days = 5

full_chapters = chapters // days

print(full_chapters)

Output:

Five goes into seventeen three full times.

There are still chapters left over.

To find those, we need another operator.

10. Remainder — Modulo

Python uses:

to calculate the remainder after division.

This is called the modulo operator.

Example:

chapters = 17
days = 5

remaining = chapters % days

print(remaining)

Output:

Because:

17 = 5 × 3 + 2

the remainder is:

This operator becomes extremely useful in programming.

11. Exponents

Python uses:

**

for exponentiation.

Example:

result = 2 ** 3

print(result)

Output:

Because:

2 × 2 × 2 = 8

You will not use exponents constantly in beginner programs, but you should recognize the operator.

12. Arithmetic Operator Reference

Operator	Meaning
`+`	Addition
`-`	Subtraction
`*`	Multiplication
`/`	Division
`//`	Floor division
`%`	Remainder
`**`	Exponent

You do not need to memorize this table instantly.

You will learn the operators by using them.

13. Numbers Inside Variables

Arithmetic becomes much more useful when combined with variables.

Example:

total_chapters = 50
completed_chapters = 18

remaining_chapters = total_chapters - completed_chapters

print(remaining_chapters)

Output:

Trace what happened:

total_chapters → 50
completed_chapters → 18

Then:

50 - 18

produces:

That result is assigned to:

remaining_chapters

14. Reassignment With Numbers

Variables containing numbers can also be reassigned.

Example:

chapters_read = 3

print(chapters_read)

chapters_read = 5

print(chapters_read)

Output:

3
5

The same reassignment rule from Lesson 1.1 still applies.

The most recent assignment determines the variable’s current value.

15. Arithmetic Using Existing Values

Consider:

chapters_read = 3

chapters_read = chapters_read + 2

print(chapters_read)

This may look strange at first.

Trace it carefully.

Before the second line:

chapters_read → 3

Python evaluates the right side:

chapters_read + 2

which becomes:

3 + 2

which becomes:

Then Python assigns:

back to:

chapters_read

The output is:

The = symbol is assignment.

It is not saying:

3 equals 5.

It is saying:

Calculate the right side, then store the result using the name on the left.

16. Order of Operations

Consider:

result = 2 + 3 * 4

print(result)

Does Python calculate:

2 + 3

first?

No.

Python follows an order of operations.

Multiplication happens before addition.

So:

3 * 4 = 12

then:

2 + 12 = 14

Output:

17. Parentheses Change the Order

Now consider:

result = (2 + 3) * 4

print(result)

The parentheses tell Python to calculate:

2 + 3

first.

So:

2 + 3 = 5

then:

5 * 4 = 20

Output:

Compare:

2 + 3 * 4

with:

(2 + 3) * 4

They contain the same numbers and operators.

But they produce different results.

18. A Useful Order of Operations Model

For the arithmetic in this course, think in this order:

Parentheses
Exponents
Multiplication and division
Addition and subtraction

Operators at the same level are generally evaluated from left to right.

Do not rely on guessing.

When the intended order might be unclear, parentheses can make your meaning easier to understand.

19. Calculating Percentages

Suppose a reading plan contains:

40 chapters

and you have completed:

10 chapters

To calculate the percentage completed:

completed ÷ total × 100

In Python:

total_chapters = 40
completed_chapters = 10

percentage = completed_chapters / total_chapters * 100

print(percentage)

Output:

25.0

That means:

25%

of the reading plan is complete.

20. Parentheses Can Make Formulas Clearer

We could also write:

percentage = (completed_chapters / total_chapters) * 100

The result is the same.

But the parentheses make the intended calculation easier for a human reader to see.

Readable code matters.

You are not only writing instructions for Python.

You are also writing code that humans need to understand.

21. Trace the Program

Consider:

total_chapters = 60
completed_chapters = 15

remaining = total_chapters - completed_chapters
percentage = completed_chapters / total_chapters * 100

print(remaining)
print(percentage)

Trace it.

Step 1

total_chapters → 60

Step 2

completed_chapters → 15

Step 3

60 - 15 = 45

So:

remaining → 45

Step 4

15 / 60 = 0.25

Then:

0.25 * 100 = 25.0

So:

percentage → 25.0

Final output:

45
25.0

22. Predict It

Do not run these until you make your prediction.

Predict It #1

total = 20
completed = 8

remaining = total - completed

print(remaining)

What is printed?

Predict It #2

result = 5 + 2 * 3

print(result)

What is the result?

Which operation happens first?

Predict It #3

result = (5 + 2) * 3

print(result)

How is this different from Predict It #2?

23. Predict It — Division

Consider:

chapters = 17
days = 5

print(chapters / days)
print(chapters // days)
print(chapters % days)

Before running it, predict all three results.

Then compare your prediction with Python.

24. Build It — Bible Reading Progress Calculator

Create a file named:

reading_progress.py

Use these supplied values:

Total chapters: 50
Completed chapters: 20

Create variables for both values.

Then calculate:

Chapters remaining
Percentage completed

Display both results.

Your Program Is Complete When

Your calculations produce:

30
40.0

The first value represents chapters remaining.

The second represents percentage completed.

Your program must:

Store the total in a numeric variable
Store the completed amount in a numeric variable
Calculate the remaining chapters
Calculate the completion percentage
Store the calculated results in variables
Print the results

Do not manually store:

or:

40.0

as the answers.

Python must calculate them.

Hint Ladder

Hint 1

Remaining chapters require subtraction.

Hint 2

Percentage completed follows:

completed / total * 100

Hint 3

Store each calculated result in its own descriptive variable before printing it.

25. Fix It

A student wants to calculate remaining chapters.

Their code is:

total_chapters = 40
completed_chapters = 12

remaining = completed_chapters - total_chapters

print(remaining)

The program prints:

-28

The arithmetic works.

But the logic is wrong.

Ask:

Which value should be subtracted from which?

Fix the calculation.

26. Fix the Order

A student wants to find the average of two study-session lengths:

20 minutes
40 minutes

They write:

average = 20 + 40 / 2

Python does not calculate the intended average.

Why?

Which operation happens first?

Use parentheses to make the intended calculation explicit.

27. Break It — Strings Versus Numbers

Consider:

chapters = "5"

Compare it with:

chapters = 5

One stores text.

The other stores a number.

Do not try to solve conversion problems yet.

That belongs in Lesson 1.5.

For now, simply explain:

Which value is a string?
Which value is a number?
Which one is intended for arithmetic?

This distinction will become extremely important.

28. Explain It

Answer these in your own words:

What is an integer?
What is a float?
Why is "25" different from 25?
What does / do?
What does // do with positive numbers?
What does % tell you?
What does ** do?
Why does multiplication happen before addition?
What do parentheses allow you to control?
How can you calculate a percentage?

29. The Narrow Path — Common Mistakes

Putting numeric values in quotation marks

This:

chapters = "20"

stores text.

This:

chapters = 20

stores a number.

If the value is intended for arithmetic, quotation marks matter.

Reversing subtraction

If you want chapters remaining:

total - completed

is not the same as:

completed - total

Think about what the calculation represents.

Forgetting order of operations

This:

20 + 40 / 2

does not mean:

(20 + 40) / 2

Python follows its arithmetic rules.

Use parentheses when necessary.

Confusing `/` and `//`

These operators do different things.

performs normal division.

//

performs floor division.

Do not use them interchangeably.

Forgetting what `%` means

The % operator does not calculate a percentage by itself.

In Python arithmetic:

returns a remainder.

Percentage calculations are formulas that normally involve division and multiplication by 100.

30. Check Your Understanding

Answer these without looking back if possible:

What is an integer?
What is a float?
Is "10" a number or a string?
Which operator performs multiplication?
Which operator performs normal division?
Which operator performs floor division?
Which operator calculates a remainder?
Which operator calculates an exponent?
What happens first in 2 + 4 * 3?
How can parentheses change an expression?
How would you calculate the percentage of a reading plan that has been completed?
Why are descriptive numeric variable names useful?

⚖️ Put It on the Scale

Use this supplied data:

Total chapters: 66
Completed chapters: 18
Reading days remaining: 7

Without introducing any concepts not taught in this lesson:

Store all three values as numbers.
Calculate the number of chapters remaining.
Calculate the percentage completed.
Calculate how many full chapters could be evenly assigned to each remaining day using floor division.
Calculate how many chapters would remain after that division using modulo.
Predict every result before running the program.
Run the program.
Compare the actual results with your predictions.

Do not search for a completed solution.

Use the operators you learned.

Graduation Standard

Do not move forward until you can store integers and floats, perform arithmetic with variables, use +, -, *, /, //, %, and **, control calculations with parentheses, and calculate a percentage.

You can now make Python perform calculations using information stored in variables.

Next, we will make our programs interactive.

Instead of changing values inside the source code ourselves, we will let the person running the program provide information.

Lesson 1.4 — Input and Output

Learning Goal

1. The Need

2. Numbers Are Different From Strings

3. Integers

4. Floats

5. Addition

6. Subtraction

7. Multiplication

8. Division

9. Integer Division

10. Remainder — Modulo

11. Exponents

12. Arithmetic Operator Reference

13. Numbers Inside Variables

14. Reassignment With Numbers

15. Arithmetic Using Existing Values

16. Order of Operations

17. Parentheses Change the Order

18. A Useful Order of Operations Model

19. Calculating Percentages

20. Parentheses Can Make Formulas Clearer

21. Trace the Program

Step 1

Step 2

Step 3

Step 4

22. Predict It

Predict It #1

Predict It #2

Predict It #3

23. Predict It — Division

24. Build It — Bible Reading Progress Calculator

Your Program Is Complete When

Hint Ladder

Hint 1

Hint 2

Hint 3

25. Fix It

26. Fix the Order

27. Break It — Strings Versus Numbers

28. Explain It

29. The Narrow Path — Common Mistakes

Putting numeric values in quotation marks

Reversing subtraction

Forgetting order of operations

Confusing / and //

Forgetting what % means

30. Check Your Understanding

⚖️ Put It on the Scale

Graduation Standard

Put It on the Scale. ⚖️🐍📖

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Confusing `/` and `//`

Forgetting what `%` means