The method
For A:B = C:X, cross multiply: A × X = B × C, so X = B × C ÷ A.
Examples
- Recipe: 2 cups of rice need 3 cups of water. For 5 cups of rice: X = 3 × 5 ÷ 2 = 7.5 cups of water.
- Map: 1 cm = 25 km. A 7.2 cm distance is 7.2 × 25 = 180 km.
- Price: 3 kg of onions cost ₹120. 5 kg cost 120 × 5 ÷ 3 = ₹200.
Check your answer
Divide each side: 2 ÷ 3 should equal 5 ÷ 7.5. Both give 0.667, so the answer is right.
Try it yourself. Get your own result in seconds with the free Ratio Calculator.
Open the Ratio CalculatorFrequently asked questions
What is cross multiplication?
Multiplying the outer and inner terms of two equal ratios to find a missing value.
Does order matter in a proportion?
Yes. Keep the same units in the same positions on both sides.
More guides
Written and reviewed by the editorial team at Evavel Private Limited. Found an error? Email care@calculations.one.