Why does my calculator give a different answer for 1 + 2 × 3?

It depends on the calculator. A scientific calculator follows the order of operations (PEMDAS), so 1 + 2 × 3 = 7. A basic calculator works through each step as you enter it, so it adds 1 + 2 first and shows 9. Neither is broken; they follow different rules.

Which rule does math class use?

Homework follows the order of operations: parentheses, exponents, multiplication and division (left to right), then addition and subtraction (left to right). So 8 − 2 × 3 = 2, not 18.

The 5-second test

Enter 1 + 2 × 3 = on any calculator.

Result	What it does
7	Follows the order of operations
9	Calculates left to right, step by step

How to check homework answers without getting caught out

1. Run the test above first.
2. On a left-to-right calculator, do the multiplications and divisions yourself first, then add the results.
3. Read each number back before you press equals. Most wrong “checks” are a typo, not a math mistake.

Checking out loud

I build Voice Calculator. It has both modes: Basic works left to right (1 + 2 × 3 = 9) and Scientific follows the order of operations (1 + 2 × 3 = 7). A student can say the problem aloud and see each number on the tape before the answer appears, then tap Read to hear the whole line back. It checks arithmetic only; it doesn’t show steps or explain the method.

Related: why a voice calculator should show what it heard, calculator help for dyscalculia and the Voice Calculator guides.