Question Video: Multiplying Decimals by Integers Mathematics • 5th Grade

2.58 × 4 = _.


Video Transcript

2.58 multiplied by four equals what.

One way of finding the answer to this problem could be to use doubling to help. We know that doubling is the same as multiplying by two. And so, doubling twice is exactly the same as multiplying by four. And even if we’re not sure how to double a decimal like 2.58, we could even remove the decimal point and think of the number as 258. As long as we count the number of decimal places at the end and put it back in. So doubling then doubling again could be a method we could use to find the answer.

Another method we could use to find the answer, and this will be the method we’ll go through, is short multiplication. So we can start by writing the number that we’re multiplying, 2.58. Then underneath, we’ll write the number that we’re multiplying by, four. And then we need to go through each of the digits in 2.58, one by one, and multiply them by four. But before we start, we want the digits in our answer to have the correct value. And to make sure that we do this, we need to draw a decimal point exactly underneath where the other decimal point is. This way we know our digits are all going to have the correct value.

Firstly, we need to multiply the hundredths by four. Eight times four is 32. So eight hundredths multiplied by four is thirty-two hundredths. We can write this as two in the hundredths place. And then exchange 30 of our hundredths for three-tenths. Next, we multiply the tenths digit by four. We know that five fours are 20. So five-tenths multiplied by four equals twenty-tenths. We already have three-tenths that we’ve exchanged. So that takes us to twenty-three tenths. So we can write the digit three in the tenths place and exchange 20 of those tenths for two ones.

Finally, we need to multiply our ones digit by four. Two times four equals eight. We have two ones underneath that we’ve exchanged. So that takes us to 10 ones. We can write a zero in the ones place and exchange the 10 ones for one 10. To multiply this decimal number by a whole number, we used short multiplication. 2.58 times four equals 10.32.

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