Proportion calculator
In a proportion a : b = c : x, cross multiplication holds: a times x equals b times c. The missing term follows in one step, x = b × c ÷ a, and most school-level scaling reduces to that single rule.
x
20
- Ratio
- 2.5
Rates and thresholds
- Cross multiplication
- a × x = b × c
- Missing term
- x = b × c ÷ a
How do I use cross multiplication?
Multiply the terms that sit diagonally and divide by the fourth. If 3 kilos of apples cost €6, then 5 kilos cost 6 × 5 ÷ 3 = €10. Keep the units in the same order on both sides of the equation — kilos against kilos, euros against euros.
How do I scale a recipe?
Find the factor by which the servings change and multiply every ingredient by it. A recipe for four scaled to six means a factor of 6 ÷ 4 = 1.5, so 200 grams of flour becomes 300. Baking time does not follow that factor — it depends on the shape of the tin, not the quantity.
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