At time $\small{t=0}$, a tank contains $\small{15}$ oz of salt dissolved in $\small{100}$ gallons of water. Then brine containing $\small{2}$oz of salt per gallon of brine is allowed to enter the tank at a rate of $\small{5}$ gal/min and the mixed solution is drained from the tank at the same rate.

(a) How much salt is in the tank at an arbitrary time? oz.

(b) How much salt is in the tank at time $\small{30}$ min? oz.