The combustion of acetylene is $2C_2H_2 + 5O_2 \rightarrow 4CO_2 + 2H_2O$. From stoichiometry, 2 volumes of acetylene require 5 volumes of oxygen. So, 100 cc of acetylene requires $(5/2) \times 100 = 250$ cc of oxygen. Since air is 20% oxygen, the volume of air required is $250 / 0.20 = 1250$ cc.