a woman drives to work and averages 70 kilometers per hour for the trip. On her trip from work to home she averages 100 kilometers per hour. What is her average speed for the round trip?

Answer 1

average speed = total distance/total time

we don't know how far it is to her place of work,
let that distance be x

time on first part = x/70
time for return trip = x/100
total time = x/70 + x/100
= 17x/700

avg speed = 2x/(17x/700)
= 2x(700/17x)
= 1400/17 km/h
or appr 82.4 km/h