Final answer:
For every 15 X atoms, there are 3 Y atoms in the compound X₃ Y.
Step-by-step explanation:
The compound has the formula X₃ Y. For every 15 X atoms, there are 3 Y atoms. This is because the formula X₃ Y indicates that there are 3 Y atoms for every 1 X₃ group. Since there are 3 X atoms in each X₃ group, for every 15 X atoms, there will be 15/3 = 5 X₃ groups and therefore 5 Y atoms.