Oxidation Numbers?

O.N. = oxidation number

KMnO₂ :
O.N. of K = +1

For neutral compound, sum of O.N. = 0
(O.N. of K) + (O.N. of Mn) + (O.N. of O)×2 = 0
(+1) + (O.N. of Mn) + (-2)×2 = 0
O.N. of Mn = +3


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HIO₂ :

For neutral compound, sum of O.N. = 0
(O.N. of H) + (O.N. of I) + (O.N. of O)×2 = 0
(+1) + (O.N. of I) + (-2)×2 = 0
O.N. of I = +3


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K₂Cr₂O₇ :

For neutral compound, sum of O.N. = 0
(O.N. of K)×2 + (O.N. of Cr)×2 + (O.N. of O)×7 = 0
(+1)×2 + (O.N. of Cr)×2 + (-2)×7 = 0
(O.N. of Cr)×2 = +12
O.N. of Cr = +6