The odds, , of an event, , is defined by:

where is the probability of occuring, and we say the odds are in favor of the event.

For example, lets say you knew the probabilty for some event occuring is . What are the odds in favor of such an event? Since

The odds in favor are 1/2 : 1, or to clear fractions, multiply both sides by 2 and say the odds in favor for A are 1 : 2 (it's more likely for A not to occur, twice as likely for it to not occur than occur)

Now, apply this to solving your problem. What can you come up with?