#2
Top 50 Mistake
Adding the percentage instead of multiplying
"The Add-On Error"
The Mistake in Action
Increase £80 by 15%.
Wrong: $80 + 15 = £95$
Why It Happens
Students see "increase by 15" and instinctively add 15 to the number. They're treating the percentage as an absolute amount rather than a proportion of the original.
The Fix
15% means "15 per hundred" — it's a fraction of the amount, not £15.
Correct method: Step 1: Find 15% of £80 $$15\% \times 80 = 0.15 \times 80 = £12$$
Step 2: Add to original $$80 + 12 = £92$$
Or use the multiplier: $$80 \times 1.15 = £92$$
Spot the Mistake
Can you identify where this student went wrong?
Increase £80 by 15%
$80 + 15 = £95$
Click on the line that contains the error.
Related Topics
Learn more about the underlying maths: