13 and 19 are prime.
33 and 45 are composite.
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If you can arrange the counters in the shape of a rectangle with at least two in each row and each column then the number is composite. The numbers of row and the numbers of columns are factors of the given number. If the only rectangle you can make is the "degenerate" one, with only one row or only one column, then the number is prime.
43 is prime. 32 is composite.
4 is composite, 31 is prime.
There are four of each.
33 has the factors ,3, & '11' ; it is composite 43 has no factors ; it is a prime number 63 has the factors 3,7,9,21 ; it is composite.