How to use it
- Choose Direct if both quantities grow together (price and weight, distance and fuel) or Inverse if one grows while the other shrinks (workers and days, speed and time).
- Enter A and the value B that goes with it.
- Enter the new value C. X is shown at once.
Formulas
| Proportion | Relation | X |
|---|---|---|
| Direct | A / B = C / X | X = B × C ÷ A |
| Inverse | A × B = C × X | X = A × B ÷ C |
In a direct proportion B ÷ A is the same for every pair (for example the price of one kilogram); in an inverse one the product A × B stays the same (for example the total worker-days).
Worked examples
- Direct. 3 kg of apples cost 150. How much are 5 kg? X = 150 × 5 ÷ 3 = 250, at 50 per kilogram.
- Fuel. A car uses 7.5 l per 100 km. For 340 km: X = 7.5 × 340 ÷ 100 = 25.5 l.
- Inverse. At 60 km/h a trip takes 2 hours. At 80 km/h: X = 60 × 2 ÷ 80 = 1.5 hours.
Common mistakes
- Choosing direct for an inverse case. More workers do not take more days. Ask whether doubling A doubles B (direct) or halves it (inverse).
- Mixing units. Keep A and C in the same unit (both kilograms or both grams), otherwise X is off by a factor of 1000.
- Assuming a proportion where there is none. Taxi fares with a fixed start fee or bulk discounts do not grow in strict proportion.