The first step is ALWAYS to write down the formulas you'll use
to calculate the answer. The formula you choose has to be one
that connects the information you have with the answer you need.
For this problem, you know the distance she drove and the time it
took her, and that information has to somehow give you her average
speed.
Here's the perfect formula for the job:
Average speed =
(total distance covered) / (total time to cover the distance) .
Now, technically, we can't calculate the average speed for the
whole trip, because the total time would have to include her time
in the store. The best we can do is calculate the average speed
during the driving time, going and returning.
total distance =
(7 miles going) + (7 miles returning) = 14 miles
time to cover the distance =
(20 minutes going) + (15 minutes returning) = 35 minutes
Average speed while driving =
(14 miles) / (35 minutes)
= ( 14/35 ) (mile/minute)
= 2/5 mile/minute
or
(2/5 mile/min) x (60 min/hr)
= ( 120/5 ) (mile/hr) = 24 mile/hour