Find the missing value in a proportion. Enter A, B, and C, and the calculator finds X in A : B = C : X, using cross-multiplication.
Two ratios that are equal form a proportion.
Cross-multiplying turns the equation into a simple multiplication.
Dividing by the known term isolates the missing value.
Proportion: A ÷ B = C ÷ X
Cross-multiply: A × X = B × C
Solve: X = B × C ÷ A
A recipe uses 2 cups of rice for 3 people. How much for 6 people?
A (rice): 2
B (people): 3
C (rice needed): 6
Set up: 2 : 3 = 6 : X
Cross-multiply: 2 × X = 3 × 6 = 18
Divide: X = 18 ÷ 2 = 9
Answer: X = 9 people can be served
What is cross-multiplication?
In A/B = C/D, multiplying diagonally gives A × D = B × C. This removes the fractions, and leaves a simple equation to solve.
Why can't a denominator be zero?
A ratio with zero on the bottom does not make sense, because you cannot divide by zero. The calculator rejects this instead of giving a wrong answer.