Video: Evaluating Numerical Expressions by Multiplying Rational Numbers

Isabella works as a library assistant. She earns $9.75 per hour. How much will she get paid if she puts in 40 hours of work a week?

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Video Transcript

Isabella works as a library assistant. She earns nine dollars 75 cents per hour. How much will she get paid if she puts in 40 hours of work in a week?

In order to calculate the total pay, we need to multiply the rate per hour by the number of hours worked. In this question, we need to multiply nine dollars 75 by 40. There are lots of ways of approaching this calculation. One way would be to split 40 into 10 multiplied by four. We could then multiply 9.75 by 10 and then our answer by four.

Multiplying any number by 10 moves all the digits one place to the left. So we’re left with 97.5 multiplied by four. Doubling 97.5 gives us 195, and we know multiplying by four is the same as doubling and doubling again. Therefore, 97.5 multiplied by four is equal to 390. This means that nine dollars 75 multiplied by 40 is equal to 390 dollars. This is Isabella’s total pay for the week.

As nine dollars 75 is just short of 10 dollars, we could start with this amount first. 10 dollars multiplied by 40 hours is equal to 400 dollars. Nine dollars and 75 cents is 25 cents less than 10 dollars. Multiplying 25 cents by 40 gives us 10 dollars as 0.25 multiplied by 40 is 10. This would be an alternative method to show that nine dollars 75 multiplied by 40 is 390 dollars as 400 minus 10 is 390.

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