The complete question is:
What is the ratio of Al3+ ions to S2- ions in a neutral compound?
Call x the number of atoms of Al(3+) and y the number of atoms of S(2-). The ratio of Al(3+) to S(2-) is x/y
Then, the neutral condition implies x(3+) + y(2-) = 0
=> 3x = 2y
=> x/y = 2/3
And the ratio is 2/3
Answer: 2/3
