[jerm174]

Natural gas is a mixture of hydrocarbons, primarily methane and ethane. A typical mixture might have Xmethane=0.915 and Xethane=0.085. Let's assume that we have 15.50g of natural gas in a volume of 15.00L at a temperature of 20.0'C.
1) How many total moles of gas are in the sample?
Does anyone know how to solve this? Given the mass, I can't find the moles because Im not given the molar mass. Im assuming the mole ratios have something to do with it but I can't figure it out. I tried to derive a system of equations to solve for a variable, but there are 3: n_total, n_methane and n_ethane.
I know n_m/n_t + n_e/n_t = 1.
n_m + n_e = n_t.
15.5 = n_m/M_m + n_e/M_e
There are additional questions, but if I can get help with this part, I will be capable of answering the rest. Thanks!

[Borek]

You have three unknowns and three equations. Just solve.