a train consists of n carriage, each of which may have a defect with probability p.all the carriage are inspected
, independentely of one another,by two inspectors, the first detects defects(if any) with probability p1 and
the second with probability p2. if none of the carrage is found to have a defect,the train departs. find the probability
of the event; ' the train departs with at least one defective carriage.