first you need to know how many moles of oxygen every reactant needs:
CH
4+2O
2 CO
2+2H
2O
C
2H
4+3O
2 2CO
2+2H
2O
C
3H
8+5O
2 3CO
2+4H
2O
now we combine them in one equation but divide each of the gases to its ratio:
0.4CH
4+0.24C
2H
4+0.16C
3H
8+(0.4*2)O
2+(0.24*3)O
2+(0.16*5)O
2+0.2CO
2 0.4CO
2+(2*0.24)CO
2 +(3*0.16)CO
2+(0.4*2)H
2O+(0.24*2)H
2O+(0.16*4)H
2O
now you need to add the mole ratios of oxygen together then you will get how many moles you need to burn 1 mole of the gas, and as you know there is 20% of oxygen in air so you can calculate the moles of air needed
check the answer cause i might be wrong.