We want to know how many people use a 200 liter container. To do that, we do (size of container)/(amount of water people drink)=200/0.5=400 people/container. Next, since we have 1 container / 400 people (1 container = 400 people, so it can go either way) , we want to find how many containers we need for 7000 people. To cross the amount of people out, we multiply the expression by 7000 people to get 7000 containers/400=2.5 containers.