hi yac678,
There is no way to tell what their market share is for the keyword. The best way is to estimate based on leaked AOL data from several years ago (Linked to below). Unfortunately, This data isn't as accurate as it could be as:
It is from AOL, not Google
It is a few years old
This is the best info available now though. I would guess that these percentages would become more skewed towards the number one position.
http://www.seobook.com/google-serp-ctr-data-search-rankhttp://www.webuildpages.com/jim/click-rate-for-top-10-search-results/