I say urgent because of the horribly small lecture I received on this section, a whole 3 minutes or so of examples. While I won't give further context I can say without a doubt I am completely lost. Here is the problem I am stuck on. In a string of 12 Christmas tree light bulbs, 3 are defective. The bulbs are selected at random and tested, one at a time, until the third defective bulb is found. Compute the probability that the third defective bulb is the: a) Third Bulb Tested b) Fifth Bulb Tested c) Tenth Bulb Tested I am at a lost. My only feeble attempt at a solution has been: (1/12)*(1/11)*(1/10), which I know for a fact is wrong. If someone could shed some light, any light, on how to tackle problems like this I would be eternally grateful.