Initially half of the balls are odd. The first ball being odd has a probability of 1/2. Taking one odd ball out leaves 4 odd and 5 even left in the bag, what is the probability of taking an odd ball out now? How do you combine the probabilities of two different events?
Another approach . . .
Ten balls numbered 0, 1, 2, ... , 9 are in a box, and two balls are taken at random.
The probability that both numbers are odd is equal to:
There are: possible outcomes.
There are: ways to get two odd numbers.
Therefore . . .