i cant understand a bit of eudiometry plz help??

Question

for example:7.5 ml of a gaseous hydrocarbon was exploded with 36ml of oxygen.The volume of gases on cooling was found to be 28.5ml,15ml of which was absorbed KOH and the rest was absobed in a solution of alkaline pyrogallol.if all volumes are measured under the same conditions,deduce the formula of the hydrocarbon?

Answer 1

15 ml is CO2 (15/7.5 = 2 atoms of C per molecule)

13.5 ml is O2

So, 22.5 ml of O2 was used. (7.5 ml goes into water, so 7.5 ml of H2)

The gas is Ethyne C2H2.