A tank holds 40 kg of brine that is 5% salt by weight. How many kilograms of fresh water must be poured in so that the brine becomes 2% salt?

Only water is being added, so the salt stays fixed at 5% of 40 = 2 kg. For 2 kg to be 2% of the tank, the tank must hold 2/0.02 = 100 kg, so 100 − 40 = 60 kg of water must go in. (100 is the FINAL weight of the brine, not the water added; and halving the strength from 5% to 2% is not a matter of adding an equal 40 kg — that would give 2/80 = 2.5%.)