Formulas and rules

Every key math formula and rule, organised by topic and school level, with examples and study tips.

Fractions 6

Divide the numerator and denominator by their HCF.

a/b + c/d = (ad + bc) / bd

Find a common denominator, then add the numerators. Never add denominators.

a/b x c/d = ac / bd

Multiply numerators together and denominators together.

a/b / c/d = a/b x d/c

Keep, flip, multiply: keep the first fraction, flip the second and multiply.

a b/c = (a x c + b) / c

A mixed number has a whole part and a fraction. Convert to an improper fraction to calculate.

1/2 = 0.5 = 50%

Divide the numerator by the denominator for a decimal; multiply by 100 for a percentage.

Printable cheat sheet