The minimum number of moles of O _ 2 required for complete...

(1) Combustion of propane.

${C}_{3}{H}_{8} + 5{O}_{2}\left(g\right) \to 3{CO}_{2}\left(g\right) + 4{H}_{2}O1mole5mole$

(2) Combustion of butane

${C}_{4}{H}_{10} + \dfrac{13}{2}{O}_{2} \to 4{CO}_{2}\left(g\right) + 5{H}_{2}O1mole\dfrac{13}{2}mole$

so total mole of ${O}_{2}$ $= 5 + 13 = 18$