Hello,

First of all, translate all the information given in the text. Then you may have useless information, but you cannot know at first sight.

(No, the fact that the boy's name is Colin is not an important information )

Note that y=800-x.Why ?

Let s be Colin's speed.

We know that the speed is proportional toy. This means that there exists a constant k such that .

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Let's deal with "x against the time"

Remember that the speed corresponds to the derivative of the distance with respect to the time.

Thus

, where k'=800k.

So you're now with the following equation :

I hope you know how to solve a differential equation