Question

Explain why $3\times 5\times 7+7$ is a composite number.

Answer 1

$\mathrm{Let}a=3\times 5\times 7+7=7\left(3\times 5+1\right)=7\times 16$
It can be seen that a has two more factors 7 and 16 other than 1 and the number itself.
Therefore, it is a composite number.