How long will it take to drive 30 miles going 15 mph?

Distance=speed times time

distance=30mi
speed=15mph
t=t

so
30=15 times t
divide both sides by 15
2=t

2 hours

It will take two hours to drive 30 miles. 

mph= Miles per hour 
 The first hour you're going 15 miles, then another hour later you're at 30 miles.

Two hours.