Q.Write the set of all prime numbers less than 20 in roster form.

List every whole number less than 20 that has exactly two distinct positive divisors (1 and itself).

Checking each number from 2 to 19 for primality gives the complete list.

Step 1 — Recall the definition of a prime number. A prime number is a whole number greater than 1 with no positive divisors other than 1 and itself.

Step 2 — Check every number from 2 to 19. $2, 3, 5, 7, 11, 13, 17, 19$ each have no divisors besides 1 and themselves. The remaining numbers in that range ($4, 6, 8, 9, 10, 12, 14, 15, 16, 18$) each factor into smaller numbers (e.g. $4 = 2 \times 2$, $9 = 3 \times 3$, $15 = 3 \times 5$), so they are excluded.

Step 3 — Collect the result in roster form.