#34
Top 50 Mistake
Mixing up percentage and decimal forms
"The Percentage Pickle"
The Mistake in Action
The probability of winning a prize is 15%. Write this as a decimal.
Student writes: "15% = 1.5"
Why It Happens
Students know they need to remove the percentage sign but aren't sure how to convert. Some divide by 10 instead of 100, or remove the sign without any calculation.
The Fix
Percentage to decimal: Divide by 100 (move digits 2 places right)
$15\% = 15 ÷ 100 = 0.15$
Decimal to percentage: Multiply by 100 (move digits 2 places left)
$0.15 × 100 = 15\%$
Quick check: 15% is less than half, so the decimal must be less than 0.5. Is 1.5 < 0.5? No! So 1.5 can't be right.
Spot the Mistake
Can you identify where this student went wrong?
Convert 15% to a decimal
15% = 1.5
Click on the line that contains the error.
Related Topics
Learn more about the underlying maths: