If in the previous problem only 25.9 grams of water vapor were formed how much methane actually reacted with oxygen?

Methane and oxygen react according to the equation:CH4 + 2O2 -->CO2 + 2H2O

Thus 1 mole of methane produces 2 moles of water.

25.9 g of water is 25.9/18 moles = 1.44 moles.So we must have started with 0.72 moles of methane, which is 0.72 x 16 g= 11.51 g of methane.