whats the answer to this question?

Question

For the set of whole numbers from 1 to 20 inclusive,Tammy knows that some numbers are divisible by 5 and some numbers are odd.She is going to write each number on a different ball and place the balls in a box. If one ball is randomly selected from the box,what is the probability,to the nearest tenth,that the number written on it is divisible by 5 or is an odd number?

my work:

odd numbers=3,5,7,9 P(divisible by 5 U odd number) =P(divisible by 5)+ P(odd number)-P(divisible by 5 U odd number) 3/20+5/20-7/20 =1/20

Answer 1

There are a total of 10 odd numbers from 1 to 20 (1,3,5,...17,19). There are a total of 4 numbers which are divisible by 5 (5,10,15,20). The intersect is 2 (5 and 15). You can add all the probabilities and subtract the intersection. 10/20 + 4/20 - 2/20 =3/5 or 0.6.

This makes sense, since you are expecting a probability of over 50%, since more than 50% of the numbers are odd + divisible by 5 than not. Hope this helps