Colin sets off for school, which is 800m from home. His speed is proportional to the distance he still has to go.

Let x meters be the distance that he has gone, and y meters be the distance he still has to go.

a) Sketch the graph of x against t and y against t.

I need help with writing an expression for this word problem in both formats so that I can sketch the graph. I am bad with word problems (Worried)