How it is calculated
Direct: X = B × C ÷ A Inverse: X = A × B ÷ C- Direct: as the first grows, the second grows in the same ratio (quantity and price).
- Inverse: as the first grows, the second shrinks (workers and time to finish).
Worked example
Direct: 3 kg cost 45 — what do 5 kg cost? X = 45 × 5 ÷ 3 = 75. Inverse: 4 workers finish a job in 12 days — how long for 6? X = 4 × 12 ÷ 6 = 8 days.
Frequently asked questions
Direct or inverse?
Ask: if I double the first, does the second double (direct) or halve (inverse)?
Does it work for percentages?
Yes: if 200 is 100%, what % is 50? X = 100 × 50 ÷ 200 = 25%.
What is cross multiplication?
The same thing — solving a/b = c/x by multiplying across.