A(0)=50. Using the initial condition gives

I get

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.