4, 6, 8, 9, 10, 12, 14, 15, 16, 18, 20, 21, 22, 24, and 25.
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The sum of the first 5 numbers is 15. 1+2+3+4+5=15
The sum of the first 15 positive even numbers is 240. (Simply square 15, then add 15 to the result: 15 x 15 = 225. 225 + 15 = 240).
The first 15 composite numbers are 4, 6, 8, 9, 10, 12, 14, 15, 16, 18, 20, 21, 22, 24, and 25.
The first six triangular numbers are 1, 3, 6, 10, 15 and 21.
You cannot find the first 15 prime numbers by dividing by 2 and 3. This method would not identify 25 as being a composite.