You've been given a rate for water consumption by 7 men over 2 weeks. If there are now three times as many men, and they need water for twice as long, you can see that it will require 6 times as much water, right?

More formally - the rate of consumption of water per man-week is 13 Qt per 2 weeks per 7 men, or: . Given 4 weeks and 21 men, the amount of water needed is: