Redox equations with only one overall reactant/Product...?

[Destiny]

They're driving me up the wall.
I have two questions from the Heinemann Workbook 2 that look like this: 

I googled for help, but one website said that I should just duplicate the product or reactant for both half equations.  So that method worked for the first equations, where I duplicated the S204(2-) ion.  But it doesn't work for the second questions, I attempted to duplicate HCOOH(aq) and it didn't work with the O2

The answers say this:

It looks as if for the second question O2 simply didn't react with anything, and became nothing, which they then balanced :/
Could you guys give me any tips as to how to approach these sorts of questions?  Would be much appreciated....

[ggxoxo]

Yea... lol I saw the qn in Heinemann 2!!!

[Destiny]

I still don't get it...

[ggxoxo]

Ask your teacher...

[Destiny]

I will, she's an examiner Dx