Number

Number is the bedrock of IGCSE Maths. These are reliable marks, and the skills here — especially percentages, standard form and bounds — reappear inside almost every other topic.

Factors, multiples, HCF and LCM

Write each number as a product of prime factors (a factor tree), then:

HCF (highest common factor) = product of the primes they share.
LCM (lowest common multiple) = product of the highest power of every prime that appears.

Worked example. $24 = 2^3 \times 3$ and $36 = 2^2 \times 3^2$. Shared: $2^2 \times 3 = 12$, so $\text{HCF} = 12$. Highest of each: $2^3 \times 3^2 = 72$, so $\text{LCM} = 72$.