Since Mr. Williiams travels at 30 mph, in after time t (in hours) have elapsed, he will have traveled the distance d
d = 30t
Mr. Speedster starts out at the same point 4 hours later at 60 mph. So after time t have elapsed since Mr. Speedster started, he will have traveled for t - 4 hours and covered the distance 60(t-4).
At the time t when Mr. Speedster overtakes Mr. Williams, they will have traveled the same distance d:
d = 30t = 60(t - 4)
Solve this equation for t:
30 t = 60t - 240
t = 8
8 hours have elapsed since Mr. Williams started traveling, so 8 - 4 = 4 hours have elapsed since Mr. Speedster started traveling.
Mr. Speedster will overtake Mr. Williams in 4 hours.
This means Mr. Speedster will overtake Mr. Williams in 8 hours after