Combustion of octane is:
C8H18 + 25/2O2 -> 9H2O + 8CO2
You are given 10 moles of octane and you are required to find the volume of octane
10 moles C8H18 (25/2 moles O2/ 1 mole C8H18) = 125 moles O2
V = 125 moles O2 ( 24 dm3 / 1 mole of gas)
V = 3000 dm3