Question

A student goes to college,driving his car at 20 km per hour. He returns though same route ,driving at 40 km per hour . Find average speed during entire journey





distance d from home to college
total distance = 2 d

time to college = d/20
time to get back = d/40

total time driving = d/20 + d/40

average speed = 2d/(d/20+d/40)

= 80/(2+1) = 80/3 = about 27 km/h

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