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Lesson 1 of 2

SHORTCUT

Cross multiply

Two fractions equal to each other? Multiply diagonally.

The question

Solve 3/4 = x/20

The trick

When one fraction equals another, multiply each top by the opposite bottom and set the results equal.

Worked example

  1. 1Multiply diagonally: 3 × 20 = 4 × x
  2. 260 = 4x
  3. 3Divide both sides by 4: x = 15

So: x = 15

Remember it like this

Draw an X across the equals sign. The two arms of the X multiply together.

In the exam

Only cross multiply when it is exactly one fraction equals one fraction. If there is anything added on either side, combine it into a single fraction first.

Why it works

Multiplying both sides by both denominators clears the fractions completely. The diagonal pattern is just the shortcut for doing both multiplications at once.

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Try it yourself

0/3 correct
  • 1

    Solve 2/5 = x/25. What is x?

  • 2

    Solve 3/7 = 12/x. What is x?

  • 3

    Solve x/6 = 10/4. What is x?