Hello, I'm working on a related rates problem that I can't seem to get. It should be fairly easy, but trying to piece the values together just isn't working out for me.

The question is:

A railroad bridge is 20 m above, and at right angles to, a river. A person in a train travelling at 60 km/h passes over the centre of the bridge at the same instant that a person in a motorboat travelling at 20 km/h passes under the centre of the bridge. How fast are the two people separating 10 seconds later?

Thank you in advance for any help!