A milk tanker runs the 180 km from a chilling centre to the city at a steady 60 km/h. Coming back along the same road it runs at 45 km/h and also stands for 30 minutes at a checkpost. Its average speed over the whole round trip (in km/h) is:

Out: 180/60 = 3 hours. Back: 180/45 = 4 hours of running plus half an hour standing = 4.5 hours. So 360 km takes 7.5 hours — an average of 48 km/h. (Equivalently the return leg is worth an effective 180/4.5 = 40 km/h, and for equal distances $2\times 60\times 40/100=48$.) Ignoring the halt and using $2\times 60\times 45/105$ gives 51.43; the plain average of 60 and 45 gives 52.5; and taking the plain average of 60 and the effective 40 gives 50 — plain averages of SPEEDS are wrong whenever the two legs take unequal times.