Number
One of the largest areas at foundation and one of the smallest at higher, which is why the same topic can feel very different depending on the tier.
- Fractions, decimals and percentages used interchangeably, including as multipliers
- Percentage change, reverse percentages, simple and compound interest, growth and decay
- Indices and roots, including negative and fractional indices at higher tier
- Standard form, in calculations as well as in notation
- Rounding, estimation, and error intervals; upper and lower bounds at higher tier
- Exact calculations with π on both tiers; surds at higher tier
A price rises 20% and then falls 20%. Is it back where it started?
No — 1.2 × 0.8 = 0.96, so it is 4% lower. Multipliers make that obvious; adding and subtracting percentages hides it.
Where it usually goes wrong Reverse percentages. "A coat costs £68 after 15% off, what was the original price?" is not £68 plus 15%. It is £68 ÷ 0.85, and it appears on every paper.