A Tank Contains 200 Gallons of Water in Which 300 $A ( t )$

Multiple Choice

A tank contains 200 gallons of water in which 300 grams of salt is dissolved. A brine solution containing 0.4 kilograms of salt per gallon of water is pumped into the tank at the rate of 5 liters per minute, and the well-stirred mixture is pumped out at the same rate. Let $A ( t )$ represent the amount of salt in the tank at time t. The correct initial value problem for $A ( t )$ is

A) $\frac { d A } { d t } = 2 + A / 40 , A ( 0 ) = 0.3$
B) $\frac { d A } { d t } = 2 - A / 40 , A ( 0 ) = 0.3$
C) $\frac { d A } { d t } = 5 + A / 40 , A ( 0 ) = 300$
D) $\frac { d A } { d t } = 5 - A / 40 , A ( 0 ) = 300$
E) $\frac { d A } { d t } = 0.4 - A / 40 , A ( 0 ) = 300$