(a) Brine.

Brine is flowing into a tank containing 50 lb of salt dissolved in 300 gallons of water. The salt concentration of the brine is 2 lb/gal, the input rate is 3 gal/min. The well-stired mixture is draining from the tank at a rate of 3 gal/min. Let A(t) be the amount of salt in the tank (in lb) at time t.