Part (i) is correct
Part (ii), I guess you are talking about average kinetic energy,
not theory...
The answer is 3/2 K T, where K is the boltzmann's
constant
Part (iii)
pV = nRT, from here, u see that R is constant, T and V is same
for both helium and nitrogen. Only difference is n.
So, we can expect that helium molecules will exert twice the amount
of pressure as the nitrogen molecules in this question
Hence, ans is 80 kPa.