(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 to y. 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.
, where k'=800k.
So you're now with the following equation :
I hope you know how to solve a differential equation