CH4 + 2(O2) → CO2 + 2(H20)

If we look up the atomic weights of the atoms that make up octane and oxygen, we get the following numbers:

Carbon (C) = 12.01

Oxygen (O) = 16

Hydrogen (H) = 1.008

So 1 molecule of methane has a molecular weight of: 1 12.01 + 4 1.008 = 16.042

One oxygen molecule weighs: 2 * 16 = 32

The oxygen-fuel mass ratio is then: 2 32 / 1 16.042 = 64 / 16.042

So we need 3.99 kg of oxygen for every 1 kg of fuel

Since 23.2 mass-percent of air is actually oxygen, we need : 3.99 * 100/23.2 = 17.2 kg air for every 1 kg of methane.

So the stoichiometric air-fuel ratio of methane is 17.2.