probabilities

chaps,
all this mathematics on probabilities of having a success with a number of wells is just a little overcomplicating things. let us look at the question this way. in any outcome the probability is 1. if a well has a pos of 10% then it has a probability of failure of 90%. In that case the outcome of 1 minus the probability of failure is 1-90% equals 10% which is the pos. trivial but it sets the scene. now consider a second well also with a pos of 10%. the first well has a 90% chance of failure, and so does the second therefore the chance of both failing is .9 times .9 or .81 meaning 81%. the chance of success is therefore 1 minus .81 equals .19 meaning 19%. if we have a third well also pos 10%, then the chance of all three failing is .9 times .9 times .9 equals .73 meanng 73%. the chance of success is therefore 1-.73 or 27%.
you can keep this going so long as you like and with different probabilities of success if required.
it is intuitive that the more tests the better the chance of an outcome even if each test has only a fixed pos, and this methodology shows how this works. i should add that the resultant probability of success (pos), eg 27% in the 3 well case includes the pos of 1 success plus the pos of 2 successes plus the chance of all 3 successes.
to conclude i would comment that if one multiplies the pos values, eg in the 3 well case .1 times.1 times.1, this gives .001 or .1% which sounds horrible. but you should recall that this is the probability of all 3 wells succeeding. which again intuitively we would not feel to be very likely. really hope this is informative and clarifying.