Common Subtraction Mistakes to Avoid
Even experienced mathematicians make subtraction errors. Knowing which mistakes are most common helps you avoid them before they happen.
Mistake 1 – Forgetting to Borrow
Wrong
52 − 37: 2−7 impossible so writes 5 without borrowing → 25 (wrong)
Correct
Borrow from tens: 12−7=5, 4−3=1 → 15
Mistake 2 – Subtracting the Larger from the Smaller Digit
62 − 47: (wrong) 7−2=5, 6−4=2 → 25. (right) borrow → 52−47=15
Mistake 3 – Misaligning Decimal Points
5.3 − 2.14 written without padding: misread as 5.3 − 2.14 ≠ 3.19. Correct: 5.30 − 2.14 = 3.16
Mistake 4 – Sign Errors with Negatives
(−5) − (−3) wrongly becomes −8. Correct: −5 + 3 = −2
Mistake 5 – Not Simplifying the Denominator When Subtracting Fractions
3/4 − 1/3 written as 2/1 (wrong). Must find LCD=12 → 9/12 − 4/12 = 5/12
Mistake 6 – Ignoring the Minus Sign When Expanding Brackets in Algebra
(5x + 2) − (3x − 1) wrongly gives 2x + 1. Correct: 5x + 2 − 3x + 1 = 2x + 3
Checklist
- Always check: did I borrow correctly?
- Align decimal points before subtracting.
- Distribute the minus sign in algebraic subtraction.
- Use addition to verify: answer + subtrahend = minuend.
- Estimate first to catch wildly wrong answers.
Practice: Spot and Fix the Mistake
A student's incorrect answer is shown below. Work out the correct answer. If you get it wrong (or leave it blank), we'll fix the mistake together.
Find the correct answer.