Differential Equations · Superchallenge problem

A tank starts with $50$ L of pure water. Brine with concentration $4$ kg/L flows in at $2$ L/min, and the well-mixed solution drains out at $2$ L/min. Find the time $t$ (in minutes, to two decimal places) at which the salt content first comes within $1$ kg of its long-run steady-state value.