Explanation

Calcium (Ca) typically has an oxidation number of +2.

Oxygen (O) typically has an oxidation number of -2.

In the chlorite ion (ClO2-), let the oxidation number of Cl be 'x'.

The total charge of the chlorite ion is -1.

So, the equation becomes: x + 2*(-2) = -1

Solving for x: x - 4 = -1, therefore x = +3

Since there are two chlorite ions in the compound, the overall oxidation number of Cl in Ca(ClO2)2 is +3.