Mixed Concentration Problem, please revise.

I was tutoring and I wrote the solution to this mixed problem, but I do not have the same as the back of the text, please check and tell me what I do wrong.

Question:

A 400-gal tank initially contains 100 gal of brine containing 50 lbs of salt. Brine containing 1 lb of salt per gallon enters the tank at the rate of 5 gal/s, and the well-mixed brine in the tank flows out at the rate of 3 gal/s. How much salt will the tank contain when it is full of brine?

Attached is the solution that I wrote.

Thank you!!!

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.