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fantasybreeze7
Because it can be divided by 3 and 17. Prime means it is only divided by 1 and itself (example would be 3). That's why 51 is composite.
"A number is considered composite if it has factors that are not one and itself, 51 happens to equal 17 x 3, making it not prime and therefore composite. 51 mod 3 is 0.""

So, yes, 51 is composite.