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 :).
Because just like compounds, the table of chemicals can be looked at as having all the different pieces of it. So the different chemicals are the different "elements" to the periodic table.
