You can solve this by saying that there are 3 Cl for every 1 Al therefore a ratio of 1:3. If you have 12 Cl ions you can find how many Al ions there is
so (x)Al : 12 a unkown amount of Al ions gives you 12 Al ions from the ratio we know that we have 1 Al for 3 Cl therefore we can divide 12 by 3 to get the number of Al ions. The answer is 4 Al ions for 12 Cl ions.
Hope this helps :).