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Video: Estimating Products with Decimals

Kathryn Kingham

Isabella’s shower uses 7.8 liters of water per minute. By rounding the number of liters to the nearest whole number, estimate how many liters she will use if her shower lasts 9 minutes. Calculate exactly how many liters of water a 12-minute shower will consume.

03:33

Video Transcript

Isabella’s shower uses 7.8 liters of water per minute. By rounding the number of liters to the nearest whole number, estimate how many liters she will use if her shower lasts nine minutes. Then calculate exactly how many liters of water a 12-minute shower would consume.

In the first part of this problem, we need to estimate how many liters Isabella will use in a nine-minute shower. It tells us to do this by rounding to the nearest whole number. We take our 7.8 liters. We need to round this number to the nearest whole number.

To round to the nearest whole number, we’ll look at the value in the tenths place, here an eight. Because the value in the tenths place is larger than four, we need to round 7.8 up. 7.8 liters to the nearest whole number is eight liters.

We’ve estimated that Isabella’s shower uses eight liters per minute, and we want to know how much water she’ll use in a nine-minute shower. So we multiply eight liters per minute times nine minutes. Eight times nine equals 72. At this rate, a nine-minute shower would use about 72 liters of water. This is only half of the problem.

The second half says calculate exactly how many liters a 12-minute shower would consume. Here we use our 7.8 liters per minute and multiply that value by 12 minutes. Starting all the way on the right, we multiply two times eight equals 16. Bring down your six; carry your one. Then we multiply two times seven, which equals 14. But we can’t forget the one we carried over. 14 plus one is 15.

From there, we go back to our tenths place, and this time we’ll be multiplying by one. One times eight equals eight. One times seven equals seven. We can add our two partial products. Six plus zero equals six. Five plus eight equals 13. Write down our three; carry our one. After that, we have one plus seven plus one, which equals nine. But we can’t forget our decimal place.

Because we have a decimal point one digit to the left in our multiplication problem, we’ll also have a decimal place one digit to the left in the solution. At a rate of 7.8 liters per minute, for a 12-minute shower, Isabella would use 93.6 liters of water.