The oxide of a metal contains $40\%$ of oxygen. The valency of metal is 2 . What is the atomic weight of the metal ?

 $100 g$ of metal oxide will contain $40 g$ oxygen and $60 g$ metal.

Therefore, moles of oxygen will be $\frac{40 g}{16 g/mol}=2.5 mol$

It is given that the valency of metal is 2. Since we know that oxygen has 2 valancy therefore the formula of metal oxide will be MO.

Hence, number of moles of metal $=$ number of moles of oxygen $=2.5 mol$

Thus the weight of the metal =$\frac{60}{2.5}=24 g/mol$

Hence the correct option is (A).