If an element, X, can form an oxide that has the formula X2O3, then element X would most likely be located...

on the Periodic Table in the same group as
(1) Ba (3) In
(2) Cd (4) Na

The answer is In, but why?

O has a 2- charge. x 3 = 6-
2 x X = 6+ so X = 3+
Elements with a 3+ charge are in column 13 along with B, Al, and In.

When you have a molecular formula and you want to determine the charges of the elements within the formula, take the numbers (in this case 2 and 3) switch them, and place them above the element. So the 2 after the X would go above the O (meaning oxygen has a 2- charge) and the 3 would go above the X (X has a 3+ charge) Since X has a 3+ charge it is in the Group 3. Elements that have a 2+ charge are found in group 2 and elements that have a 1+ charge are in group 1.

Here's an example of the switching method. If you have the Molecular Formula Mg3N2, and you want to find the charge of each element. The 2 would go above the Mg, so you know the Mg has a 2+ charge. And the 3 would go above the N, so you know N has a charge of 3-