text: The conditional probability is the likelihood of an event, given the occurrence of another event.  It is usually written,\n\np(A|B)\n\nwhich is read as "the probability of A, given B."  This conditional probability assumes B has happened.\n\nIn your problem, assuming Micron goes UP, then on 10 of 17 days, Gateway goes up.  Thus the probability of Gateway going up, given Micron going up, is 10/17.\n\nSimilarly, p(Gateway up | Micron not up) = 2/8.\n\nThe key is, you take the first event as given and go from there.