The mass percent of oxygen in the compound is

A sample of metal weighing 2.30 g is combined with oxygen; the metal oxide weighs 3.01 g.

The metal, without oxygen, weighs 2.3 g. When it's oxidized, it's new mass is 3.01 g. The additional weight, presumably, is oxygen.

3.01 g - 2.30 g = 0.71 g Oxygen

0.71 g O / 3.01 g metal oxide = 0.2359 x 100% = 23.59% Oxygen

((3.01 g-2.30 g)/3.01 g)*100 = __._ %

Took a chemistry class in 1973