average speed= distance traveled/total travel time

Question

if you go the exact same distance to and from work but average 30 miles an hour getting there and 40 miles an hour coming home what is your average speed for the round trip?

Answer 1

t_go=d/30

t_ret=d/40

ave speed=2d/(d/30+d/40)=2/(1/30+1/40)

=2(1200)/70=240/7=34 2/7 mph