"No. of moles of Oxygen" (n_(O_2))="Mass of Oxygen in gram"/"Molar mass of Oxygen"
=(64g)/(32g/"mol")=2" mol"
"No. of moles of Nitrogen"(n_(N_2))="Mass of Nitrogen in gram"/"Molar mass of Nitrogen"
=(14g)/(28g/"mol")=0.5" mol"
"No. of moles of Carbon-dioxide" (n_(CO_2))="Mass of Carbon-dioxide in gram"/"Molar mass of Carbon-dioxide"
=(66g)/(44g/"mol")=1.5" mol"
So
"mole % of Oxygen in the mixture"=(n_(O_2))/(n_(N_2)+n_(O_2)+n_(CO_2))xx100
=2/(2+0.5+1.5)xx100=2/4xx100=50%