A gaseous hydrocarbon upon combustion gives 0.72g of water and 3.08g of CO2. The empirical formula of the hydrocarbon is :

18g of H2O contains 2g of H. So, 0.72g of water contains 0.72 × 2/18 = 0.08 g of H.
44g of CO2 contains 12g of C. So, 3.08g of CO2 contains 3.08 × 12/44 = 0.84 g of C.
Number of moles of C = 0.84/12 = 0.07
Number of moles of H = 0.08/1 = 0.08
C : H = 0.07 : 0.08 = 7 : 8
Therefore, the empirical formula is C7H8.