Question

How many prime numbers up to $100$ have $3$ at their ones place?

Answer 1

According to the question, we have
numbers between $1$ to $100$ having unit digit $3$ is
$3,13,23,33,43,53,63,73,83,$ and $93$
Clearly, we can see that $33,63,$ and $93$ is divisible by $3$
So, except these three numbers all are prime numbers
$3,13,23,43,53,73,$ and $83$ are prime numbers
Therefore, number of primes between $1$ and $100$ having unit place digit $3$ is $7$.