[Nutsinatin]

I'll take the bait...

Cyclohexane will have the chemical formula C6H12
It's Mr will be C6(12x6)= 72 + H12(1x12) = 12 so Mr = 84gmol^-1
So the Mr of cyclohexane will be 84gmol^-1
If n moles = Mass/Mr then n = 0.420/84= 5x10^-3mol
So we have 5x10^-3mol of cyclohexane
As the Avogadro constant is per mole of substance we need to multiply the number of moles of cyclohexane by the constant. Therefore 5x10^-3 x 6.0x^23 = 3x10^21
So we have 3x10^21 molecules of cyclohexane.
Each cyclohexane molecule contains 12H atoms, so we have 12 times as many H atoms as cyclohexane molecules, therefore 3x10^21 x 12 = 3.6x10^22
So we have 3.6x10^22 hydrogen atoms in 0.420g of cyclohexane.

I hope I'm right else I'll have to hang my head in shame for that one, although I'm not sure how clear my workings are, might have to tidy that one up when I'm not so tired!