11You can find the answer to this question by the following link and also its structure.
http://books.google.co.in/books?id=0OcDAAAAMBAJ&pg=PT37&lpg=PT37&dq=the+no.+of+S-S+bond+in+S3O9&source=bl&ots=2xG6vR05KG&sig=YILrXGIWfLGCFaLhvaojZGZRaaE&hl=en&ei=kIWGTN-9OY-SuwP14JGKBA&sa=X&oi=book_result&ct=result&resnum=6&ved=0CCgQ6AEwBQ#v=onepage&q=the%20no.%20of%20S-S%20bond%20in%20S3O9&f=false
see the question no. 13 and its answer given in the hints after all the question gets over.
:)
