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?

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.