40% is the answer to this question. In order to solve this, you need to do some computations. You need to find the ratio of the mass of oxygen and the mass of the compound. You need to multiply this by 100. The given has a mass of 24 then add it to 16. It will become 16/40.
This can still be simplified into lowest terms which will be 2/5. If it would be transformed into decimal, it will be .4. This would be multiplied by 100 like mentioned earlier so the answer will be 40%. You can use the same formula if in case you would come across similar questions.
Solve the problem by first finding the ratio of the mass of oxygen to the mass of the compound; then multiply by 100. MgO has a mass of 24 + 16. Percent by mass of O = 16 / 40.
Now reduce to lowest terms: 2 / 5 or .4 and multiply by 100. 16 / 40 x 100% = ((40%)).