Question 7
In 160 liters of 12% salt solution, there is 160 * 0.12 liters of pure salt. That's 160*0.12 = 19.2 liters of salt. To make it a 20% solution, you have to have 5 times more than than, that is, 96 liters. Since you have 160 liters now, you need to evaporate 160-96 liters, or 64 liters of water.