To find the thickness of one paper, you set up proportions. 10,000 sheets/85 cm=1 sheet/ x cm. Then you cross multiply and get,
85=10,000x
Then find x,
x=.0085
Or .0085 cm
Another way to find it is just to divide 85 by 10,000 from the start because you need to find the thickness of one paper, not ten thousand.
Hope this helps.