Multiplying a decimal by a whole number

Multiplying a decimal by a whole number uses the same method as multiplying two whole numbers.

Question

Calculate:

a) 2.43 \times 7

b) 2.4 \times 5

Diagram of multiplying a decimal by a whole number

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.

Diagram of multiplying a decimal by a whole number

b) There was one digit after the decimal point in the question (4), 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.

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.
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Check that you have the correct answer by finding an approximate solution.

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

{2}\times{5}={10} and {3}\times{5}={15}, so you are looking for an answer which is slightly higher than {10}. So an answer of {12} seems correct.