Just add them together but leaving out 17, 19, 23 and 29 which are prime numbers
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The composite numbers in that range are 10, 12, 14, 15, 16, 18, 20. I don't know if you're including 10 and 20 or not. Add them up however you like.
composite numbers between 11 and 21:12 14 15 16 18
The sum first Prime number is (2 + 3 + 5 + 7 + 11 + 13 + 17 + 19 + 23 + 29 ) = 129 The sum of the first composite numbers is (4 + 6 + 8 + 9 + 10 + 12 + 14 + 15 + 16 + 18) = 112 The difference is 129 - 112 = 17 !!!!!
Composite numbers between 6 and 20 are 8, 9, 10, 12, 14, 15, 16, and 18.
It is: 12+14+15+16+18 = 75