Answer:
This question seems to be incomplete, and sadly after an online search i couldn't find the complete question, so i will answer in a general way how to solve this type of problem.
Ok, suppose that we have N elements, and we select one at random, so all of them have the exact same probability of being selected, so if N elements have the same probability of being selected, the probability for each one will be 1/N.
Now suppose that in those N elements, there are K elements with a particular characteristic, such that K < N.
Then if we want to find the probability of getting one of these K elements, the probability will be K times the probability of each single element, that is 1/N, then the probability is:
P = K*(1/N) = K/N.
I know that this may be hard to follow, but let's see a quick example, there are 6 marbles in a bag, and 2 of them are red, which is the probability of picking one red marble at random?
Here we have N = 6, K = 2
Then the probability is:
P = 2/6 = 1/3 = 0.33
Now, in our problem, we have 120 people, and we want to find the probability that the selected person is a mole.
The problem here is that we do not know the number of moles, then assuming that there are K moles, the probability of selecting at random a mole is:
P = K/120.
