You can ride a bicycle 15 mph. Your sister rides 12 mph. You give a one hour head start. In how many hours will you catch up if neither of you stops?
Both of you would travel the same distance, at different speeds for different times.
d = 15t = 12(t+1)
15t = 12t + 12
15t - 12t = 12
3t = 12
t = 4hrs <<<<<<
15mph * 4hrs = 60mi
12mph * (4hrs + 1hr) = 12mph * 5hrs = 60mi