Hi Vikash!

As you mentioned that oxide of metal has 40% by mass of oxygen, means 40 grams of oxygen. So, amount of metal 'm' in metal oxide= 60 grams.

Therefore, number of moles of metal 'm' = 60/24 =2.5

And number of moles of oxygen = 40/16 =2.5

Hence the ratio of moles of 'm' and O is 1:1

So, the empirical formula is MO.

Thankyou!