To find X and Y for a given number just factor the number. X is the number of primes up to the largest factor. Y is the product of the missing primes.
For example 66 = 2*3*11
The primes up to 11 are 2,3,5,7,11 so there are X=5 primes, the missing ones are 5 and 7 so Y=35.
see also CHARLIE'S & DejMar's posts.
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