3. A natural number greater than 1 that can only be evenly divided by 1 and itself is called a prime number, such as $2,3,5,7,11,13$, etc. Please circle all the prime numbers in the following number table:
Solution: All the circled prime numbers:
$$
2,3,5,7,11,13,17,19,23,29,31,37,41,43,47,53,59,61,67,71,73,79
$$