Question 519224
Firstly if you evaporate 10% solution the concentration will increase. It wont decrease.

The other way is possible

initial conc 2 % final 10%

water to be evaporated be x 
 
when x evaporates there is 64 - x left,
which has 90% water

2% of 64 = 1.28 oz is salt,
64 - 1.28 = 62.72oz is water.
 
62.72 - x = 0.9 (64 - x)
 62.72 - x = 57.6 - 0.90x
 -0.1x = -5.12
 x = 51.2 oz of water

m.ananth@hotmail.ca