A milk van leaves the dairy at 9 a.m. and drives to a town at a steady 30 km/h. A pickup truck leaves the same dairy for the same town at 10 a.m., and the two reach the town together at 1 p.m. The truck's speed (in km/h) is:

The van is on the road from 9 a.m. to 1 p.m., four hours, so the town is $4\times 30=120$ km away. The truck does that same 120 km in three hours, i.e. at 40 km/h. (The truck has three-quarters of the time, so its speed must be 4/3 of the van's; raising 30 by a quarter to 37.5 km/h uses the ratio the wrong way round and would land the truck there at 1.12 p.m., while 45 km/h would bring it in 20 minutes early.)