A car drives east at 40km/h and then turns back and drives west back to the strating point at 70 km/h, what is the avarge speed of the car?
well, let us assume he drives 280 km out.
he takes 280/40 outward or 7 hr
he takes 280/70 back or 4 hr
so he takes a total of 11 hours to go 560 km
so
560/11 km/hr
(you are allowed to pick any other distance. I grabbed a common multiple because I do not have a calculator handy :)