A car traveled from A to B at a speed of 15 km/h and return at a speed of 30 km/h. Find the average speed for the whole journey.

Answer 1

time for first part = d/15
time for 2nd part = d/30
total time = d/15 + d/30
= d/10

avg speed = total distance/total time
= 2d/(d/10)
= 20 km/h

notice we can't just average the two speeds.

Answer 2

Best to imagine a distance first. For eg, a distance of 30km.

Time for first part = 2 hours
Time for second part = 1 hour
Total time = 3 hours.

avg speed = total distance/total time
= (30+30)/(2+1)
= 60/3
= 20 km/h