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)