A neglected large lawn contains a certain type of weed. The area of the lawn covered by the weed at time t years is Wm². The rate of increase of W is directly proportional to W.

Write down a differential equation that is satisfied by W.

OK now i know the answer is [dw/dt = KW] because these questions go by the same format all the time

BUT

I need to know WHY it is that. I don't want any nasty surprises when i go into my exam :/