4. Mary had to run to catch the train, got off two stops later and then walked to school. Which of the following speed-time graphs would best represent her journey?
Solution
E

The order of speeds needs to be train $>$ running $>$ walk. This rules out $A$ and $B$. The train section of the trip needs to have just a single intermediate stop in the middle of the trip. However $C$ shows no intermediate stop, just a slowing, and $D$ shows two intermediate stops. This leaves $E$ as the only feasible solution.