effusion rates....please help!

[Lilly]

Could someone please explain to me how to do this question...
The rate of effusion of a particular gas was measured to be 24.0 mL/min. Under the same conditions the rate of effusion of pure methane gas, (CH4) is 47.8 mL/min. What is the molar mass of the unknown gas?

[chem_tr]

Effusion rates are indirectly proportional with the molar mass; if the molecule is bigger, it can't move fast. So the particular gas has a greater molar mass.

Calculate methane's molar mass by taking C:12, and H:1 grams/mol, and find the unknown gas' molar mass.

Regards,
chem_tr