Lets try to build the largest set in which no 2 numbers are divisible by one another
What i can do is add all primes to the list and exclude 1 because 1 divides every number(and of course it's not prime)
so I Get {3,5,7,11,13,17,19} NOW FROM remaining elements that are 1,9,15 and 21 . as soon as you add anything from these numbers to the set , 2numbers get divisible
hence least size required is 7+1=8