Operators & Math

This lesson builds the app's tip calculator — the money-math heart of the Bill Splitter. Money means decimals, and decimals raise a question everyone hits: do I want to round the number, chop it, or just display it differently?

We'll start from whole-number arithmetic and work up to that, then finish with comparisons — the operators that turn numbers into decisions.

Python is a calculator

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Nothing surprising yet. Everything stays a clean whole number.

Three kinds of division

Division is where decimals first appear, and Python gives you three tools depending on what you actually want:

What does this print?

print(17 // 5)

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% is worth dwelling on: 17 % 5 is 2 because 5 goes into 17 three times with 2 left over. It's how you test whether something divides evenly — n % 2 == 0 means "n is even" — and you'll use it constantly.

Note that / gives a float even when the division is exact: 10 / 2 is 5.0, not 5.

Order of operations

Normal maths rules apply — *, /, //, % before + and -. Parentheses cost nothing and remove all doubt:

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Floats have a rough edge

A number with a decimal point is a float, and any expression involving one produces one. But floats are stored in binary, and some perfectly ordinary decimals have no exact binary form:

What does this print?

print(0.1 + 0.2 == 0.3)

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This isn't a Python bug — every language using binary floats does it. It's why you never compare money with ==, and why a receipt needs deliberate rounding.

Round, chop, or just display?

When a number has decimals there are four different things you might want, and they are genuinely different:

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You want…	Use	Result
the exact number	just use it	`16.799999999999997`
a rounded number	`round(x, n)`	`16.8`
the whole part	`int(x)`	`16`
a nice string	`f"{x:.2f}"`	`"16.80"`

The mental model: round() and int() change the value. An f-string spec only changes the display — the stored number keeps full precision.

Step through it and watch which names hold numbers and which hold text:

step through it

1price = 16.799999999999997

2rounded = round(price, 2)

3chopped = int(price)

4shown = f"{price:.2f}"

5print(rounded, chopped, shown)

Two more that catch people out:

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Percentages

A percentage is "multiply, then divide by 100". An 18% tip on an $84 bill:

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Use a variable for the percent so any rate works:

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Updating a variable using itself

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A common slip is expecting the shorthand to work before the variable exists:

broken — fix itNameError

Run it and read the error. Then give the code what it's missing so it prints 10.

Comparing values

Arithmetic makes numbers. Comparison turns numbers into True / False — which is what every decision in a program is built from. There are six:

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= assigns a value; == asks a question. Mixing them up is a rite of passage.

Comparisons work on text too, and text comparison is case-sensitive:

What does this print?

print("Cafe" == "cafe")

Combining comparisons

and, or and not let you ask compound questions:

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and is True only when both sides are.
or is True when at least one side is.
not inverts.

You'll use these constantly from the next lesson onward, where they decide which branch of your code runs.

🎯 Your turn

Write calculate_tip(bill, tip_percent) — it returns the tip in dollars, rounded to two decimals:

calculate_tip(100, 18) → 18.0
calculate_tip(84, 20) → 16.8

Hint — round(bill * tip_percent / 100, 2).

Then press ▶ Run, tap the Live App try chips to call it with different inputs, and hit ✓ Check. Green = this piece of the app is built. ✅