This is really obvious. All two digits composite numbers must have a factor less than or equal to their square root. Square root of 100 is 10. Thus all compo

This is really obvious. All two digits composite numbers must have a factor less than or equal to their square root. Square root of 100 is 10. Thus all composite numbers under 100 must have a factor under 10. There are only four primes less than 10: 2, 3, 5 and 7. Once you exclude all numbers divisible by 2, 3 and 5 these are the composite numbers left: 49, 77, 91.