I seriously have no idea how to approach this, even after staring at it for an hour.

A high-tech company purchases a new computing system whose intitial value is V. The system will depreciate at the rate f = f(t) where t is the time measured in months. Suppose that

$\displaystyle f(t) = \frac{V}{15} - \frac{V}{450}t $ if $\displaystyle 0<t\leq{30}$. If (note: if t>30, f(t) = 0)

Determine the length of time T for the total depreciation $\displaystyle D(t) = \int^t_0 f(s)ds$ to equal the initial value V.

Can somebody please help me get this set up? I'm totally frustrated.

Thanks.