Salt flows in at a rate of (5 g/L)*(3 L/min) = 15 g/min.
Salt flows out at a rate of (x/10 g/L)*(3 L/min) = 3x/10 g/min.
So the net flow rate of salt, given by
in grams, is governed by the differential equation,

which is linear. Move the
term to the right side, then multiply both sides by
:


Integrate both sides, then solve for
:


Since the tank starts with 5 g of salt at time
, we have


The time it takes for the tank to hold 20 g of salt is
such that
