A man completed a trip of 136 km in 8 hrs. Some part of the trip was covered at 15 km/hr. What is the part of the trip covered at 18 km/hr?

distance = rate * time

distance = distance

_{15kmph}+ distance_{18kmph}136 km = 15kmph * (Time at 15) + 18kmph * (Time at 18)

Let Time at 18 = x

Assume no other driving speeds

Time at 15 is 8-x

136 = 15(8-x) + 18x

136 = 120-15x+18x

16 = 3x

x = 16/3 = 5.33 hr (5hour,20min)