Let the volume of work to be proofread be x (no. of pages, no. of books - whatever)
Cyndi takes 10 hours to complete x.
So in 1 hour, Cyndi does x/10 of the work
Paul takes 11 hours to complete the same x.
So in 1 hour, Paul does x/11 of the work
In 1 hour, Cyndi and Paul together would complete (x/10 + x/11) = 
of the work.
Total work to be completed = x
So if they work together, the time taken would be =
Total work / amount of work completed in 1 hour
= 
= 
= 
or approximately 
hours.
Hope you got :)
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