Question

How much water must be evaporated from 64 ounces of a 2% salt solution to make a 10% salt solution?

Answer 1

$64 \times 2\%=64 \times 0.02=1.28$
64 ounces of a 2% salt solution contains 1.28 ounces of salt and thus 62.72 ounces of water.

The mass of the 10% solution is (x+1.28), where x is the mass of water.
1.28 divided by (x+1.28) must be equal to 10%.
$\frac{1.28}{x+1.28} \times 100\%=10\% \\ \frac{1.28}{x+1.28} \times 100=10 \\ 1.28 \times 100=10(x+1.28) \\ 128=10x+12.8 \\ 115.2=10x \\ x=11.52$

The 10% solution contains 11.52 ounces of water.
$62.72-11.52=51.2$

51.2 ounces of water must be evaporated.