A gaseous fuel mixture of 30 percent propane (C3H8), and 70 percent butane (C4H10), on a volume

Question:

A gaseous fuel mixture of 30 percent propane (C3H8), and 70 percent butane (C4H10), on a volume basis is burned in air such that the air–fuel ratio is 20 kg air/kg fuel when the combustion process is complete. Determine (a) the moles of nitrogen in the air supplied to the combustion process, in kmol/kmol fuel;
(b) the moles of water formed in the combustion process, in kmol/kmol fuel; and
(c) the moles of oxygen in the product gases.