### Why 21 is not a perfect number?

First, you want to know the definition of a perfect number: A perfect number is an integer which is equal to the sum of all its positive integer divisors that are less than itself. Example: The positive integer divisors of 6, less than 6, are 1, 2, and 3. The sum of those divisors is 1 + 2 + 3 = 6. Therefore, 6 is a perfect number. Now, 21 does not meet this definition…

### Prime two digit number divisible by 4 the product of your digits is less than 5 what number is this?

There is no such number. A prime number cannot be divisible by 4. If the product of the digits is less than 5 and there are only two digits, the possible digits could be {0, 1}, {0, 2}, {0, 3}, {0, 4}, {1, 1}, {1, 2}, {1, 3}, {1, 4}, or {2, 2}. Thus, the possible numbers are 10, 20, 30, 40, 11, 12, 21, 13, 31, 14, 41, or 22. The prime numbers among…