(S2O6)2- oxidation no

The answer is as follows:

圖片參考：http://i320.photobucket.com/albums/nn347/old-master/090316_1.jpg?t=1237219669

S has an oxidation number of 2.5

You have to view this as a whole not by just one atom/ion. S4 works as a group here with oxidation number of 10. The following weblink gives a diagram describing this:

http://www.madsci.org/posts/archives/may2001/988919069.Ch.r.html