In a sealed tank is a large sprig of elodea and a snail. the tank is kept for several weeks on a sunny window sill. both the plant and snail live and appear to be healthy. why does this happen?

Based on the scenario, both the snail and the plant are alive because the snail produce carbon dioxide while the plant produce oxygen.

In order to live, snail need oxygen. The thing is , to create an oxygen, the plant need carbon dioxide. Basically both of them provide the fuel for each other and because of that, they're still able to live in a sealed tank

hope this helps

Mg is used to measure mass