Attempt $1$: $20$ attempts remaining.

At time $t=0,$ a tank contains $20$ oz of salt dissolved in $100$ gallons of water. Then brine containing $40z$ of sale per gallon of brine is allowed to enter the tank at a rate of $5ga\frac{l}{m}in$ 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 sale is in the tank at time $20$ min $?$ oz$.$