hi all,
i would like to know whether i can / how to calculate the mean value
of occurrences from a given percentage probability for a given number
of occurrences.

for example, if know that there is a 40% chance there will be 2 or
less occurrences, how do i calculate the mean number of occurrences?

(i will then use this value as the mean part of various poisson
equations to work out the probability of various numbers of
occurrences) .

thanks for your help.