Respuesta :

31/365
This cannot be simplified because 31 is a prime number (it only has 1 and 31 as its factors) and 365 is not evenly divisible by 31.
Well, it depends on the year. Leap years have one more day. 

Regular Year:   31 : 365
Leap Year:  31 : 366 

The colons also could be slashes for fractions.