If the percentage of arsenic is 76% and the percentage of oxygen is 24% how do I calculate the chemical composition of the oxide of the arsenic?

Thanks

Pretend you have 100 grams of the stuff.

Then 76 grams of As is how many moles?
76/74.9 =1.01 moles.

How many moles is 24 grams of O? 24/16=1.5

Now take the lowest mole number, and divide it into each to get a mole ratio.
As= 1.01/1.01=1
0= 1.5/1.01=1.5
so the ratio is 1/1.5 or in whole numbers, 2/3

As₂O₃