Answer:
6/20 = 3/10
Explanation:
There are 6 numbers divisible by 3 between 1-20
Being: 3 6 9 12 15 18
With that information, we know that there are 6 numbers out of 20 that are possible to meet the criteria, hence, the chances of Mary getting a number divisible by 3 is 6/20, or 3/10 simplified.