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Decimals

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Multiplying a decimal by a whole number

Multiplying a decimal by a whole number is the same as multiplying two whole numbers. Remember:

  • If there is one digit after the decimal point in the question, there will be one digit after the decimal point in the answer.
  • If there are two digits after the decimal point in the question, there will be two digits after the decimal point in the answer.
Question

Calculate:

a) 2.43 × 7
b) 2.4 × 5

Answer
2.43 x 7 = 17.01

a) There were two digits after the decimal point in the question (4 and 3), so you must have two digits after the decimal point in the answer.

2.4 x 5 = 12

b) There was one digit after the decimal point in the question, so you must have one digit after the decimal point in the answer. The answer is therefore 12.0, but this can then be given as 12.

Check that you have a sensible answer by finding an approximate solution.

In the above example you were asked to calculate 2.4 × 5.

2 × 5 = 10, so you are looking for an answer which is slightly bigger than 10. So an answer of 12 seems sensible.

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Decimals activity

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