If isobutane and n-butane are present in a gas, then how much oxygen should be required for complete combustion of 5 kg of this gas

Question: If isobutane and n-butane are present in a gas, then how much oxygen should be required for complete combustion of 5 kg of this gas

(a) 17.9 kg

(b) 9 kg

(c) 27 kg

(d) 1.8 kg

Correct Answer: (a) 17.9 kg