Multiplying Fractions

Key concept

Multiplying fractions means multiplying the numerators, then the denominators. Then simplify if you can. Cross-cancelling divides a top and bottom by a common factor first to keep numbers smaller.

Multiplying Fractions - introduction visual

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Multiplying Fractions poster

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Flashcards

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Step-by-step method for multiplying fractions: multiply numerators, multiply denominators, then simplify the resultCross-cancelling example showing how to simplify before multiplying fractions for an easier calculationMultiplying a fraction by a whole number: 88 times 13/22 using cross-cancelling to find 52 black piano keysConverting mixed numbers to improper fractions before multiplying, with worked example and final answer

How Do We Multiply Fractions?

  • Multiply the numerators together and multiply the denominators together.
  • Then simplify the fraction, if possible.

What Is Cross-Cancelling When Multiplying Fractions?

  • You can simplify first by cross-cancelling a numerator with a denominator.
  • Cross-cancelling makes the numbers smaller and easier to multiply.

How Do We Multiply a Fraction by a Whole Number?

  • Write the whole number as a fraction over 1.
  • Then multiply as you would with two fractions.

How Do We Multiply Mixed Numbers?

  • Convert mixed numbers into improper fractions first.
  • Then multiply and simplify the final answer.

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Frequently asked questions

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Multiply the numerators together, then multiply the denominators together, and simplify if you can: 3/5 × 2/7 = 6/35. Unlike adding, you do not need a common denominator; the denominators can be different. Because the numbers grow quickly, it is easier to cancel common factors before you multiply, which is called cross-cancelling.

Cross-cancelling means dividing a numerator and a denominator by their highest common factor before you multiply, so the numbers stay small and the answer comes out simplified. For 11/24 × 36/55: 11 and 55 share 11, giving 1 and 5; 24 and 36 share 12, giving 2 and 3. Then 1 × 3 over 2 × 5 is 3/10.

Write the whole number over 1, then multiply as usual, cross-cancelling first if you can. A piano has 88 keys and 13/22 of them are black: 88/1 × 13/22. Cancel 88 and 22 by 22 to get 4 and 1, then multiply: 4 × 13 = 52 black keys. The method is exactly the same as for two fractions.

Change each mixed number into an improper fraction first, then multiply. A mixed number is a whole number plus a fraction, not a whole number times a fraction, so multiplying the parts separately gives the wrong answer. For 2 1/3 × 9/4: 2 1/3 = 7/3, cancel 9 and 3 to get 3 and 1, and 7 × 3 over 1 × 4 is 21/4, or 5 1/4.

Check every time. If you multiplied straight across, the answer often still has a common factor to cancel: 15/35 × 5/10 gives 75/350 that way, which simplifies to 3/14. If you cross-cancelled first, the answer is usually already in its simplest form, but it costs nothing to look.

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