Question 22531
One way to do this is to "follow the water".
You started with 3 gallons that was 90%water.
You boiled off 1 gallon that was 100%water.
You ended up with 2 gallons that was "x"% water.

Equation:
      0.9 (3) - 1(1) = (x)(2)
       2.7    - 1    = (x)(2)
             1.7/2  = x%
                x = 0.850
 The 2 gallons at the end is 85% water.
 So it is a 15% salt water solution.
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This time we will "follow the acid"
You start with 35 gallons that is 40% acid.
You add 7 gallons that is 0% acid.
You end up with 42 gallons that is x% acid.
Equation:
     0.40(35) + 0(7) = 42x
       x = 14/42 = 33% acid

Cheers,
Stan H.