Video: KS2-M18 • Paper 1 • Question 27

3.9 × 30 = _.


Video Transcript

3.9 multiplied by 30 equals what?

One way to work out the answer would be to multiply 3.9 by 10 and then times the answer by three. 3.9 multiplied by 10 is 39. Now we need to multiply 39 by three. First, we need to multiply 30 by three, which is 90. And nine multiplied by three is 27. 90 plus 27 is 117.

Another way to work out the answer would be to partition 3.9 into three and 0.9 or nine-tenths and then multiply each part by 30. Three multiplied by 30 is 90. And 0.9 multiplied by 30 is 27. 90 plus 27 gives us the same answer of 117.

Another way to calculate the answer would be to recognise that 3.9 is really close to four. We could round this number to its nearest whole number, which is four. Then we can multiply four by 30, which gives us 120. We added 0.1 to 3.9 to round it to four. So now we need to work out what 0.1 times 30 is and take this away from our answer. 0.1 times 30 is three. 120 subtract three is 117.

We used three different methods to calculate the answer. First, we recognise that multiplying by 30 is the same as multiplying by 10 and then multiplying by three. Our second method, we used partitioning and multiplied each part by 30. In our third method, we rounded and then adjusted. We rounded 3.9 to four and then took away 0.1 times 30, because we’d added 0.1 to 3.9. Each of these three methods gave us the same answer, 117.

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