For the first question, we calculate as follows with the basis of 100 mol dry air:
78 * 28 + 21*32 + 1*39.9 / 100
molar mass of air =28.959=28.96
For the second question, we calculate as follows:
Vapor density = molecular weight/2=14.48
Relative density= vapor density * density of hydrogen(0.089). = 1.29