Back in the 60's we did maths like this at school:

A certain council has a long straight road, and three identical snow ploughs which clear snow at a constant rate (so speed is inversely proportional to depth of snow). One day it began to snow steadily. The council sent a plough along the road. After an hour they sent the second plough behind the first. After another hour, they sent the third plough. Some time later it was still snowing steadily, and the second and third ploughs simultaneously crashed into the first. How long had it been snowing before the first plough was sent out?

You'd get 30-40 minutes for a question like that, but you'd also be able to choose a different hard question. Oh look, someone's posted the solution if you're interested:

http://www.austincc.edu/rgrmth/logic.htm#snowplow