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Explanation:
The set of primes in that given list is {2,3,5,7,11,13,17}
The set of composites is {4,6,8,9,10,12,14,15,16}. So it's basically everything but the prime numbers. We don't include the value 1 in either list since 1 is neither prime nor composite.
Counting the number of items in the list {4,6,8,9,10,12,14,15,16} leads to 9 items. This is out of 17 values total in the original list
The probability of getting a composite value is therefore 9/17
