3.

The volume of liquid after t minutes is 1 + t litres. The differential equation for the amount Q(t) of salt in the tank (in grams) at time tis $$\frac{dQ}{dt} = 2 - \frac{Q}{1+t}$$. The general solution of this linear equation is $$Q(t) = (2 t + t^2 + C)/(1+t)$$. From the initial condition Q(0) = 2we have C = 2. The concentration is Q(t)/(1+t) = (2 t + t² + 2)/(1+t)²grams/litre after t minutes. The limit of this as $t \to \infty$ is 1 gram/litre.