If proofreader 
finds 
mistakes and proofreader 
finds 
mistakes, 
of which were also found by 
, how many mistakes were missed by both 
and 
? Assume there are a total of 
mistakes, so proofreader 
finds a fraction 
of all mistakes, and also a fraction 
of the mistakes found by 
. Assuming these fractions are the same, then solving for 
gives
 |
The number of mistakes missed by both is therefore approximately
 |
