Definition and first examples of primes

Primes under $50$. $2,3,5,7,11,13,17,19,23,29,31,37,41,43,47$.

$91$ is not prime. $91=7\cdot13$, both factors between $2$ and $90$. Alternatively, $91=7\cdot13$ directly.

Conclusion. There are $15$ primes less than $50$, and $91$ is composite.