Since they pick 3 quarts in 1 hour they can pick 6 quarts in 2 hours and 9 quarts in 3 and so on. So the number of quarts they can pick in a certain amount of hours is just the total number of hours they spend picking times 3 quarts. So since they picked for 7 hours and each of those hours they picked 3 quarts, in total they picked 21 quarts