In my copy of Zill, 4th edition, this is problem 3.1.26, exactly. If that's also true for you, I think maybe your answer to (b) is incorrect. Zill did an example in the text (it's example 5 in my copy), where he gives you the solution to the problem before it starts overflowing. The solution he gives is
Double-check what you get when you plug in into this equation.
As for (c), you always have this:
How much salt is entering? How much is leaving? I don't think it's wise to continue from here, until you have the correct DE and have solved it correctly.