I make this quantity 240 atoms....
Here you specify #20.0*mol# helium atoms.... And a mole SPECIFIES #6.022xx10^23# individual particles. Why do we use it? Well, one mole, i.e. #6.022xx10^23# individual #""^1H# atoms have a MASS of #1*g#...and #6.022xx10^23# individual #""^4He# atoms have a MASS of #4*g#. And thus the mole is the link between the micro world of atoms and molecules, which we cannot see, but whose existence we can infer, to the macro world of grams, and kilograms, and litres....
#6.022xx10^23#, #-=N_A, "the Avocado number"#, is the number of particles in one mole. And so if we got #20*mol# of helium we gots...#20*molxx6.022xx10^23*mol^-1~=120xx10^23# #"helium atoms."# What is the mass of this molar quantity?
