All numerator is a series of prime number And denominator is a series of all odd numbers So answer is 13/9
And series will be 3 5/3 7/5 11/7 13/9 17/11
You see in all the cases you have only prime numbers are used say your current number is a/b then next number is next prime after a/a
13 is the next prime number after 11 so missing number is 13/11
answer is 13/11