To check if a fraction can be reduced down to lower terms follow the steps below...
Step 1: Check to see if the numerator and denominator have any factors in common
13: 1,13
20:1,2,4,5,10,20
Step 2: If they have no common factors then that means the fraction can not be reduced any lower but if they do then that means you divide the numerator and denominator by that number but in this case they do not
Answer: 13/20 is the lowest it can go
I hope this helps and if any false information was given in advance I apologize.