Re: is my answer right
What does the question ask? Read it very carefully.
Your setup is okay, but you noticed at the end that you were confused. You can save yourself such headaches by modifying your setup just a bit.
Rule #1 - Name Stuff.
You did nto actually name anything. You just popped out witht the variable 'z'. This is no good. You must WRITE DOWN what you mean. In this case, before definign 'z', I would suggest rethinking it just a bit.
Name What? What does it want? Name that!
Rather than define 'z' as the amount of mixture remaining, as you have it, define something that is EXACTLY what you want at the end of the problem. I'll use 'w', just to avoid confusion.
w = Amount of Water to be Evaporated.
See how that is exactly what the question wants? it isn't always possible to do this, particularly since the questino may have multiple parts, but it is always a good thing to think about first.
With that definition WRITTEN down, we can construct an equation - Equating the salt in each piece and describing exactly what is going on.
(36 oz)(7%) - (w oz)(0%) = (36 oz - w oz)(9%)
Solving that, you will end up with a value for 'w', you will look at your WRITTEN definition, and you will not be confused.
Note: We could have equated non-salt.
(36 oz)(93%) - (w oz)(100%) = (36 oz - w oz)(91%)
It's not to our advantage in this problem, but it is good to keep in mind that two ways generally are possible. Pick the one most suitable.
