In general, the heavier a gas molecule is, the slower it moves, and the longer it takes to effuse (escape through a tiny hole in a barrier). Graham's Law describes the relationship between the molar mass of a gas and its relative rate of effusion.$\frac {R_1}{R_2} = \sqrt{\frac{MM_2}{MM_1}}$ You know that gas 1 passed through the hole in 75 seconds, and that gas 2 took only 30 seconds. You also know that gas 2 is oxygen gas \O_2. Oxygen gas has a molar mass of 32g/mol. The only thing missing is MM1. Plug and solve