I have an assigenment where i need to calculate the lover heating value of the combustion of 100 mol of a gas mixture (MJ/Nm^{3})

The gas mixture is made of in mol%:

91.1% CH_{4}

4.7% C_{2}H_{6}

1.7% C_{3}H_{8}

1.4% C_{4}H_{10}

0.6% N_{2}

0.5% CO_{2}

the enthalpy of the combustion reaction of methane, ethane, propane and butane, are given:

Methane combustion : -804,2 kJ/mol

Ethane combustion : -1428,9 kJ/mol

Propane combustion : -2045,5 kJ/mol

butane combustion : -2663.6 kJ/mol

we are also told the gasmixture behaves like an idealgas.

The solution in the book is given to be 38,8 MJ/m^{3} but i can't seem to get the right answer no matter what i do.

my main theory to calculating it was to calculate the ΔH of the combustion reaction using the enthalpy of the 4 gas combustions given by adding them together and multiplying with their respective mol% and then dividing it with the normal volume of an idealgas 22,42 m^{3}/kmol which gives me a solution of 38,93 MJ/M^{3} which is close to the solution but not quite it.

Does anyone know how what i am missing?