Ok so this is the calculation for the probability of an event:
Divide the number of events by the number of possible outcomes. After determining the probability event and its corresponding outcomes, divide the total number of ways the event can occur by the total number of possible outcomes.
So you see for this you would need to know
1) number of possible outcomes in this case a good guesstimate
2) total number of ways the event can occur - one would need to guesstimate here
3) total number of possible outcomes - again a guesstimate
So any probability is really based upon a good guess - or the ‘Your guess is as good as mine’ theory