So the probability of E is .
If you want a formula, you can say that if the odds in favour of an event are to , then the probability of the event is .
Note also that if you're given the odds against something happening, you need to use the second number for the numerator of the fraction when you're working out the probability. For instance, if the odds are 10 to 1 against something happening, then the probability that it will happen is , and the formula is:
If the odds against an event are to , then the probability that the event occurs is .