If i Roll a 20 sided die 8 times, what are the odds I get a 19 or 20 at least once?

Question

Building a character in d20 and im looking at stats

Answer 1

In effect this is a binomial distribution with p = 2/20 = 1/10 and q = 18/20 = 9/10.

 

Probability of all 8 tosses  with no 19 or 20 = (9/10)⁸

 

Probability of 8 tosses with 1 or more occurrences of 19 or 20 = 1 - (9/10)⁸, approximately .43

 

The odds ratio 43:57