To determine if 91 is a prime number, we can check if it is divisible by any prime numbers smaller than its square root (which is roughly 9.5).
Step 1: Check divisibility by 2.
91 does not end in an even number (0, 2, 4, 6, or 8), so it is not divisible by 2.
Step 2: Check divisibility by 3.
The sum of the digits of 91 is 9 + 1 = 10. Since 10 is not divisible by 3, 91 is not divisible by 3.
Step 3: Check divisibility by 5.
91 does not end in a 0 or a 5, so it is not divisible by 5.
Step 4: Check divisibility by 7.
Let's divide 91 by 7:
7 × 10 = 70
91 - 70 = 21
7 × 3 = 21
So, 7 × 13 = 91.
Since 91 can be divided evenly by 7 and 13 (in addition to 1 and itself), it is a composite number, not a prime number.
[No]