Question

A boy runs for 10 mins at the uniform speed of 9km/h. at what speed should he run for the next 20 mins so that the average speed comes to 12km/h.

Answer 1

If the boy travels with 9km/h for 10 mins.... It means he travels a distance of 1.5 km

The total distance needed to cover if the boy have a uniform speed of 12 km/h in 30 mins is 6 km.....

So.... 6-1.5 = 4.5km

Then....

4.5 km in 20 minutes is equal to a speed of 13.5 km/h

The answer is 13.5 km/h

Answer 2

distance of 10 min =speed×time
10min=10/60hr=1/6hr
distance=1/6×9=1/2×3=0.5×3=1.5km
20min=20/60hr=1/3hr
average speed=total distance/total time
12=1.5+x/1/3+1/6
12=1.5+x/3/6=1.5+x/1/2
12=1.5+x×2
12=1.5+2x
12-1.5=2x
10.5=2x
10.5/2=2x
5.25=x
answer is 5.25km/hr