How to solve a proportion for x, and share a total in a ratio.
How to find the missing number in a : b = c : x by cross-multiplying, with any box missing, and how to share an amount out in a ratio.
By Suraj Kumar3 min read
Two ratios that are equal make a proportion. Knowing three of the four numbers is enough to find the fourth.
Cross-multiply.
In 3 : 4 = 9 : x, write both as fractions, 3/4 = 9/x. The top of each times the bottom of the other are equal: 3 × x = 4 × 9 = 36, so x = 12. The Proportion calculator shows these steps for whichever box you leave empty.
Scaling a recipe or a drawing.
A recipe for 4 that needs 300 g of flour needs x for 6: 4 : 300 = 6 : x, so x = 450 g. The same sum scales a map or a model.
Further reading
Topics: proportion calculator · solve for x · cross multiplication · ratio and proportion · share in a ratio · missing number in a ratio
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