A car covers 60 km at 30 km/h, then 80 km at 40 km/h, then 100 km at 25 km/h. What is its average speed for the whole journey, in km/h?

Average speed is total distance ÷ total time, which is a weighted average of the three speeds with each leg weighted by the TIME it takes. The times are 60/30 = 2 h, 80/40 = 2 h and 100/25 = 4 h, a total of 8 h for 60 + 80 + 100 = 240 km, so the average speed is 240 ÷ 8 = 30 km/h. Averaging the speeds as (30 + 40 + 25)/3 = 31.67 is wrong because the slowest leg lasted twice as long as either of the others, so it must count twice as heavily.