Kinetic Energy of atoms

1. The problem statement, all variables and given/known data
Write an expression E for the average kinetic energy of Helium atoms using the Boltzmann constant.

2. Relevant equations
PV = nRT

PV = [itex]\frac{1}{3}[/itex]mN[itex]\overline{c^2}[/itex]

k = [itex]\frac{R}{N_A}[/itex]

[P - Pressure ; V - Volume ; m - Mass of an atom ; N - Number of atoms ; others have their general meanings.]

3. The attempt at a solution
My problem is they're asking for an expression for the average kinetic energy of Helium atoms. It's plural. I can build an expression for the kinetic energy of a single atom.

nRT = [itex]\frac{1}{3}[/itex]mN[itex]\bar{c^2}[/itex]

[itex]\frac{3}{2}[/itex]nRT = [itex]\frac{1}{2}[/itex]mnN_A[itex]\overline{c^2}[/itex]

[itex]\frac{3}{2}[/itex]RT = [itex]\frac{1}{2}[/itex]mN_A[itex]\overline{c^2}[/itex]

[itex]\frac{3}{2}[/itex]kT = [itex]\frac{1}{2}[/itex]m[itex]\overline{c^2}[/itex] = E (Kinetic Energy of an atom)

Rest of the question is easy if this is the expression they're asking.
Do I have to take mN as the total mass of the gas and calculate it from there taking R as kN_A? That way it's making the sum a bit difficult because they haven't defined the constant N_A in the question, giving out a hint that it's not involving in this, may be.

Source: http://www.physicsforums.com/showthread.php?t=703134&goto=newpost

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