A salesman drives 60 km to a village by the hill road at 30 km/h and comes back by a longer 90 km highway at 45 km/h. His average speed for the whole outing (in km/h) is:

Each leg happens to take 2 hours, so he covers 60 + 90 = 150 km in 4 hours — an average of 37.5 km/h. (The familiar formula $2xy/(x+y)=2\times 30\times 45/75=36$ km/h applies only when the two distances are EQUAL, which they are not here; the plain average of 30 and 45 is 37.5 only by coincidence of the equal times, and it is the equal times, not the equal speeds, that make it right.)