TITLE: What is the probability of a number from 1-25 being an odd number or a factor of 20?
QUESTION [6 upvotes]: What is the probability of a number from 1-25 being an odd number or a factor of 20?
Here's my working out:

Odd numbers: 12/25 (1, 3, 5, 7, 9, 11, 13, 15, 19, 21, 23, 25)
Factors of 20: 6/25 (1, 2, 4, 5, 10, 20)
Both: 2/25 (1, 5)
P(odd number or factor of 15) = P(odd number) + P(fator of 20) -
  P(both)
= 12/25 + 6/25 - 2/25 = 16/25

16/25 was too long for MyMaths' input box so I converted it to a decimal:
16 / 25 = 0.64
That was incorrect.

REPLY [4 votes]: Thanks to the person who pointed out that I left out the 17. The answer is 0.68. It's always the simple things that I get wrong!

Odd numbers: 13/25 (1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25)
Factors of 20: 6/25 (1, 2, 4, 5, 10, 20)
Both: 2/25 (1, 5)
P(odd number or factor of 15) = P(odd number) + P(factor of 20) -
  P(both)
= 13/25 + 6/25 - 2/25 = 17/25 17 / 25 = 0.68