Differential Equations · Challenge problem

A chemical is fed into a tank at a constant rate of $5$ g/min and removed by a reaction at a rate proportional to the amount present, with constant $0.5$/min: $\dfrac{dC}{dt}=5-0.5C$, $C(0)=0$. Find the time, in minutes, at which $C$ reaches $90\%$ of its steady-state value. Give an exact answer in terms of $\ln$.