Video Transcript
Jennifer wants to burn an extra
2000 calories a week. She can choose between the
following exercise classes. Which class burns the most calories
per hour? How long would she have to spend in
that class to reach her weekly goal?
Class A lasts one and a quarter
hours and burns 600 calories. In class B, 375 calories are burned
in 45 minutes. Class C lasts one hour and burns
400 calories. In order to calculate which class
burns the most calories per hour, we need to work out the rate of change. As the three durations are
different, we can add an extra column to the table to calculate the number of
calories burned per hour.
The duration of class C is one
hour, and 400 calories are burned. Therefore, she would burn 400
calories per hour. The duration of class B is 45
minutes, which is equal to three-quarters of an hour. As three-quarters is equal to the
decimal 0.75, we could say that 45 minutes is equal to 0.75 of an hour. To calculate the rate of change or
number of calories burned per hour, we need to divide the total calories burned by
the time in hours. For class B, we need to divide 375
by 0.75. This can be done on the calculator,
giving us an answer of 500. In class B, Jennifer would burn
calories at a rate of 500 per hour.
There is an easier way of working
this out if we didn’t have a calculator. We know that in 45 minutes,
Jennifer burns 375 calories. We could therefore calculate the
number of calories burned in 15 minutes by dividing by three. 300 divided by three is 100, and 75
divided by three is 25. Therefore, Jennifer would burn 125
calories in 15 minutes. Adding 45 minutes and 15 minutes
gives us 60 minutes, and adding 375 to 125 calories gives us 500 calories. As 60 minutes is equal to one hour,
this proves that our answer for class B was correct. Jennifer would burn 500 calories
per hour.
We could use a similar method to
this for class A, starting with 600 calories in one and a quarter hours or 75
minutes. However, the quicker way to work
out the number of calories burned per hour for class A would be to divide 600 by one
and a quarter. This is the same as dividing 600 by
1.25. This is equal to 480. Therefore, Jennifer burns 480
calories per hour in class A. The largest of these three values
is 500. We can therefore conclude that
class B burns the most calories per hour.
The second part of the question
asks us how long Jennifer would have to spend in that class to reach a weekly goal
of burning 2000 calories. We can calculate the time she would
need to spend by dividing the total calories she wants to burn by the number of
calories burned per hour. We need to divide 2000 by 500. Both the numerator and denominator
are divisible by 100, leaving us with 20 divided by five. As 20 divided by five is equal to
four, Jennifer would need to spend four hours in class B to burn 2000 calories.
