A chemist has 20 liters of a solution that is 30% salt. How many liters of pure water must be added to dilute the solution to 20% salt?

Adding pure water changes the volume but not the amount of salt. The original solution has $0.30 \times 20 = 6$ liters of salt. After adding $w$ liters of water, the volume is $20 + w$ and the salt is 20% of it: $0.20(20 + w) = 6$. So $20 + w = 30$ and $w = 10$, choice A. Check: $\frac{6}{30} = 20\%$. Choice B (6) is the amount of salt, not water. Choice C (15) would give 35 liters at about 17%. Choice D (20) would double the volume and dilute the solution to 15%. Track the salt, which stays constant.