Question

A boy runs for 10 min. at a uniform speed of 18 km/hr. At what speed should he run for the next 20 min. so that the average speed comes 12 km/hr?

Answer 1

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

Average speed = Total Distance / Total Time
12 = (v1t1 + v2t2) / (t1 + t2)
12 = (18/6 + v/3) / (1/6 + 1/3)
12 = (3 + v/3) / 0.5
6 = 3 + v/3
v/3 = 3
v = 9 kmph

He should run at speed of 9 kmph for next 20 minutes.