Question

what is the oxidation number of Cl in Cl2O3

Answer 1

No charge, so the oxidation numbers should add to 0. Oxygen is usually -2, so you have:
\[(-2)\times 3 + (x) \times 2 = 0\]
Or \[-6 +2x = 0\]
And you just solve for x, which is the oxidation # of Cl.