Question

Zankrut covers three successive 2 km laps at speeds of 20 kmph, 30 kmph and 40 kmph respectively. his average speed over this distance is:

Answer 1

average speed = total distance/total time taken
total distance covered by Zankrut = 2km + 2km + 2km = 6km
total time taken is T { let }

time taken with speed 20km/h ,
t1 = 2/20 = 1/10 h [ time = distance/speed]

similarly,
time take with speed 30km/h,
t2 = 2/30 = 1/15 h

time taken with 40km/h,
t3 = 2/40 = 1/20 h

now, total time taken(T) = time taken with speed 20km/h + time taken with speed 30km/h + time taken with speed 40km/h
= 1/10 + 1/15 + 1/20
= (6 + 4 + 3)/60 = 13/60 h

now, average speed = total distance/total time taken
= 6/(13/60) = 360/13 km/h
= 27.69 km/h