Answer
Usage varied between 700 and 2700 kilowatt-hours.
Work Step by Step
An inequality is already given: $72.84\leq x \leq237.04$, where x is the cost of last year's bills. The exercise wants us to find the same inequality but in kilowatt-hours. So, since there is a fixed fee, we should subtract first from the lower and upper values:
$72.84-15.37\leq x \leq237.04-15.37$
$57.47\leq x \leq221.67$
Now, we can divide the prices by the kilowatt-hour rate to find last year's lower and upper consumptions. Since the rate is in cents, we must divide that by 100 to calculate in dollars:
$57.47/0.0821\leq x \leq221.67/0.0821$
$700\leq x \leq2700$, and that's the range in kilowatt-hours of consumption spent last year.
