In a pot there are nine blue balls numbered from 1 to 9 and also nine red balls numbered 2, 4, 6, ... to 18. Two balls are taken randomly and at the same time. Calculate the probability so that exactly one number is even?
There are only two possibilities:
1. even from the blue, and odd from the reds <---- not possible since reds are all even
2. even from the reds, and odd from the blues
= (1)(5/9) <---- 5 of the 9 balls are odd
= 5/9