Use this formula: n = m ÷ MM. You can find the molar mass in the periodic table. Recall that MM is in the units g/mol and cancels out the grams of the mass, only leaving the mols!!
n(Mg) = 5 ÷ 24.305 = 0.19758... mol
Since you gave me the answer in 2 significant figures, the answer is also in 2s.f. n(Mg) = 0.20mol
Let me know if you're confused about anything.