Question

A boy runs for 10minutes at a uniform speed of 9km/hr. At what speed should be run for the next 20minutes so that the average speed comes to 12km/hr

Answer 1

10 minutes = 1/6 hour
20 minutes = 1/3 hour

Average speed = Total distance/Total time
12 = (s1 + s2) / (1/6 + 1/3)
12 = (9/6 + v/3) / (1/6 + 1/3) ……[∵ s = vt]
6 = 9/6 + v/3
v = 3(6 - 9/6)
v = 13.5 kmph

He should travel at speed of 13.5 kmph for next 20 minutes.