Answer
$ \$2.52$.
Work Step by Step
First, change watts to kilowatts and the time to hours, since we are given the cost per kilowatt-hour.
100 W = 0.1 kW = 0.1 kilowatt.
There are 24 hours a day, and 7 days in a week, so there are 168 hours in a week.
Now find the number of kilowatt-hours used.
$$(0.1 kW)(168 hours) = 16.8 kWh$$
The cost is 15 cents per kWh, so the total is $ \$2.52$.
