The GCF of all the composite numbers under 100 is 1.
For e.g 4 & 9 are composite numbers & their GCF is 1, so if we take other composite numbers along with 4 & 9 the GCF will be 1.
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So we eliminate prime numbers. Note that the number 1 is neither prime nor composite. We have left:4689101214151618202122242526272830323334353638394042444546484950515254555657586062636465666869707274757677788081828485868788909192939495969899100
1,4,6,8,9,10,12,14,15,16,18,20,21,22,24,25,26,27,28,30,32,33,34,35,36,38,39,40,42,44,45,46,48,49,50,51,52,54,55,56,57,58,60,62,63,64,65,66,68,69,70,72,74,75,76,77,78,80,81,82,84,85,86,876,88,90,91,92,93,94,95,96,98,99,100
Not all composite numbers are even, but all even numbers except 2 are composite.
The sum of these composite numbers, including 100, is 3989.The sum of all numbers between 1 and 100 is 5,050. From this we need to subtract the sum of 1 plus all the prime numbers below 100. The sum of the primes is 1,060. Subtracting (1 + 1060) or 1,061 from 5,050 yields 3,989.
All the factors of 100 are 1, 2, 4, 5, 10, 20, 25, 50, and 100, 1,2, and 5 are prime numbers, so the composite number factors of 100 are 4, 10, 20, 25, 50, and 100.