[sn1sn2e1e2]

Given these standard reduction potentials, what is the standard reduction potential for

Co3+(aq) +3e-  Co(s)

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Given:
Co3+(aq) + e-  Co2+(aq)   E°=1.82V

Co2+(aq) + 2e-      Co(s)   E°=-0.28 V

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When you add them up to get the net equation, you get 1.54 V. But the correct answer is 0.42 V!

Am I missing something? 

[Corribus]

Reduction potentials aren't strictly additive - removing two electrons in concerted fashion, say, takes a different amount of electrons than removing them sequentially.  You have to do a specific type of conversion.