physics
posted by aleeza .
a boy walks to a school at a rate of 6km with a speed of 2.5km/h and walks back with a constant speed of 5km/h. his average speed for round trip in km/h is ?

You must mean that the rate of walking to school is 6 km/h, not 6 km.
Let the distance to school be D. (It will cancel out when you compute the avg. speed)
avg speed = (total distance covered)/(total time)
= 2D/[D/6 + D/2.5}
= 2/[(1/6 + 4/10]
= 2/[5/30 + 12/30]
= 60/17 = 3.53 km/h
Note that this is NOT the average of 6 and 2.5. He spends more time walking at the lower speed.