How many composite numbers are there from 1 to 20?

Answer 1

A composite number is a positive integer which has a positive divisor other than one or itself. In other words, if 0 < n is an integer and there are integers 1 < a, b < n such that n = a × b then n is composite. By definition, every integer greater than one is either a Prime number or a composite number. The number one is a unit - it is neither prime nor composite. For example, the integer 14 is a composite number because it can be factored as 2 × 7. Likewise, the integers 2 and 3 are not composite numbers because each of them can only be divided by one and itself. 4, 6, 8, 9, 10, 12, 14, 15, 16, 18, 20 i.e 11 composite nos between 1 to 20 included 20..... There are 11 composite numbers from 1 to 20 (including 20) viz. 4, 6, 8, 9, 10, 12, 14, 15, 16, 18 and 20.