Question

A biology teacher has 15 milliliters of a salt solution that is 65% salt. How many milliliters of water must the teacher add to make a solution that is 25% salt?

Answer 1

Assuming that water has 0% salt and that the amount of water added is x ml;

(15+x)*25 = (15+0)*65 = 975
375+25x = 975
25x = 975-375 = 600
x = 600/25 = 24 ml

The teacher must add 24 ml of water.