S is a set that satisfy
1- 1/1 is in S
2- if a/b is in S, b/2a is in S. (a, b) =1
3- if a/b and c/d are iN S , a+c/b+d is in S
Describe numbers in S
check out the first few elements
1/1 => 1/2
1/1, 1/2 => 2/3 =>3/4
1/2, 2/3 => 3/5 => 5/6
see where you can go with that