Step 1. Calculate the moles of $"O"_2$.
$32.5 cancel("g O"_2) × ("1 mol O"2)/(32.00 cancel("g O"2)) = "1.016 mol O"_2$
Step 2. Use the molar ratio to calculate the moles of $"Mg"$.
$1.016 cancel("mol O"_2) × "2 mol Mg"/(1 cancel("mol O"_2)) = "2.031 mol Mg"$
Step 3. Calculate the mass of $"Mg"$.
$2.031 cancel("mol Mg") × ("24.30 g Mg")/(1 cancel("mol Mg")) = "49.4 g Mg"$
The reaction required $"49.4 g Mg"$.