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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!!!