x= {1,2,3,8}
daca x=1 => 15/(2-1) = 15/1 = 15 apartine N
daca x=2 => 15/(4-1) = 15/3 = 5 apartine N
daca x=3 => 15/(6-1) = 15/5 = 3 apartine N
daca x=8 => 15/(16-1) = 15/15 = 1 apartine N
deci A = {1,3,5,15}
