In what I know of this problem and what you told me I think there is a 1/2 chance!
think about it, if the life span is 15 years old with standard deviation of 3 then that means we can have bats from 12~18 years old and the mean of the 50 numbers can be anything between that depending on the results we got. Then half of values in interval 12~18 is over 15 which gives it a 1/2 probability!