RatioLab

Proportion Calculator

Given three, find the fourth.

Cross-multiplication, explained

When two ratios are set equal (a/b = c/x), cross-multiplying (a·x = b·c) clears the fractions and leaves a simple equation to solve for the unknown.

Where proportions come up

Scaling a recipe up for more guests, converting map distances to real distances, and solving 'if 3 apples cost $2, how much do 7 apples cost?' are all classic proportion problems.

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RatioLab

Proportion Calculator

Given three, find the fourth.

Solve a/b = c/x → x =

Cross-multiply to find the unknown

What you entered

a: 3b: 4c: 9

1
Cross-multiply: a/b = c/x → a·x = b·c
4 × 9= 36
2
Divide by a to isolate x
36 ÷ 3= 12

Result

Solve a/b = c/x → x =: 12

Solving 3/4 = 9/x gives x = 12.

Cross-multiplication, explained

When two ratios are set equal (a/b = c/x), cross-multiplying (a·x = b·c) clears the fractions and leaves a simple equation to solve for the unknown.

Where proportions come up

Scaling a recipe up for more guests, converting map distances to real distances, and solving 'if 3 apples cost $2, how much do 7 apples cost?' are all classic proportion problems.