Now, Grand's lava fusion is used when comparing the rate of two different non reacting gases, and we're going to say here that the effusion rate of a gas at its time to travel are directly proportional. So let's say that we're dealing with gases A&B, so gas A and gas B. Since rate and time are directly proportional, that means they'll be on the same level. So the rate of gas A is equal to the time of gas A. The rate of gas B is equal to the time of gas B.
But then remember when we talked about the rate of effusion? The effusion rate of a gas and its molar mass are inversely proportional. So that would mean that the rate of gas A is on top. Therefore the molar mass of gas A would have to be on the bottom. Then the rate of gas B is here on the bottom, and therefore it's molar mass has to be here on top.
So remember, when we're directly proportional, we're on the same level with one another, and when we're inversely proportional, one will be numerator and the other will be the denominator. Understanding these relationships are key to answering questions dealing with Graham's Law of effusion.