Percentage yield and purity

A student heats 3.0 g of magnesium in excess oxygen to form magnesium oxide.

$2\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO}$

The theoretical mass of magnesium oxide is 5.0 g, but only 4.0 g is obtained.

What is the percentage yield of magnesium oxide?