Direct and Inverse Proportion

Direct and Inverse Proportion

Two quantities are in direct proportion if they increase together at the same rate — double one, double the other. They're in inverse proportion if one increases as the other decreases.

Direct proportion

If 3 pens cost £1.50, then 6 pens cost £3.00 — the cost per pen (50p) stays constant, so cost and quantity increase together.

Inverse proportion

If 4 workers take 6 days to finish a job, 8 workers (double) would take 3 days (half) — more workers means less time, in inverse proportion.

Practice

If 5 machines make 100 items in a day, how many items would 10 machines make?