A(0)=50. Using the initial condition gives
The tank initially has 100 gallons in it. It is flowing it at 5 gal/sec and out at 3 gal/sec
That means it is gaining at 2 gal/sec and has 300 gallon to go.
That will take 150 seconds.
Plugging in t=150, gives 393.75
Is that the book answer or did I make a mistake as well?. Easy to do.