If you are rolling a single six sided die numbered 1-6, you find that only 1,2,4, and 5 are factors of 20. Now the theoretical probability of an event is the ratio comparing the number of desired outcomes to the total outcomes possible.
In this case: we have 4 desired outcomes out of a total 6 outcomes possible with a single roll the die.. therefore, the probability you seek is
or approximately 67%.