A very important conversion constant to know and remember here is the Avogadro's number. It states that 1 mole = 6.022 x 10^23 atoms.
With that in mind, you can now easily solve for the moles of magnesium using dimensional analysis.
1.50 x 10^23 atoms x (1 mole/6.022x10^23 atoms) = 0.249 moles of magnesium
