Assuming that a and c are both integers, 10ac must be composite because it is equal to 2*5*a*c.
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4, composite. 67, prime.
Prime
1 is special 2 prime 3 prime 4 composite 5 prime 6 composite 7 prime 8 composite 9 composite 10 composite 11 prime 12 composite ext.
5 prime, 25 composite.
Composite