1. ## Related rates problem

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!

2. This is done like other Pythagoras related rates problems, only you have a 3-dimensional component.

$D^{2}=x^{2}+y^{2}+20^{2}$

Differentiate:

$D\frac{dD}{dt}=x\frac{dx}{dt}+y\frac{dy}{dt}$

Now, you have dx/dt, dy/dt, and you can find D once you know x and y. Find x and y after each has traveled 10 seconds. For instance, at 60 km/h, how far does the boat travel in 10 seconds. That could be your x. Carry on?.