Dr. Bob get the use PV=nRT just stuck on setting up the problem help!

PV = nRT

P is pressure in atmospheres = 6
V = volume in liters = 4.00
n = moles = grams/molar mass = 20/28.013 = ??
T = solve for this. The answer will come out in Kelvin. K = 273 + C if you want to convert K to C.