The decimal slip
An answer that's exactly 10× or 100× off isn't a maths error — it's a decimal slip: a percentage not converted (×5 instead of ×0.05), a unit boundary missed (£000s), or a calculator entry with the point in the wrong place.
Why your brain does this
Percentages, 'in thousands' footnotes and unit conversions all silently move the decimal point. The working feels identical either way, so nothing alerts you — and the test helpfully offers the 10×-off answer as an option.
The fix
Estimate first, always: "5% of 3,400 is about 170" takes two seconds and makes 1,700 impossible to accept. Read exhibit footnotes for units before computing, and treat any suspiciously round 10×-relationship between your answer and an option as an alarm, not a coincidence.
See the trap in a real question
A fund is worth £219m. Class A is 10.8% of it; within Class A, item B is 13.1%. What is the value of B?
Apply the shares in turn: multiply the total by the first percentage, then multiply that result by the second. The correct answer is £3.10m. Traps to avoid: £28.69m comes from the "sub pct on total" error; £23.65m comes from the "first level only" error; £52.34m comes from the "added the percentages" error; £1.41m comes from the "decimal slip" error.
Where it strikes
This trap appears in 47 of our questions, across: Nested proportion · Per-capita rate · Percentage of · Mental percentages · Fraction–decimal–percent.
Do you make this mistake?
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