A chemical mixing tank

At time $t = 0$ ,a tank contains $Q(0)$ lb of salt dissolved in 100 gallons of water.Assume that water containing $\frac{1}{4}$ lb of salt/gal is entering the tank at the rate of $r$ gallon/min and that the well-stirred mixture is draining from the tank at the same rate.

Let $Q(t)$ denote the amount of salt in the tank at any time $t$ and $Q(L)$ denote the amount of salt present after a very long time(infinitely long).

If $r=3$ and $Q(0) = 2*Q(L)$ ,find the time T (in minutes) after which the salt level is within $2$ % of $Q(L)$ .