Assuming that PolyU ITS is going to decide how to maintain banks of disk drives at four separate locations for security reasons, with probabilities of failure that depend on the locations. For additional security, ITS can also provide backup disk drives in each location. However, there are only FOUR backup disk drives available at all, and ITS wants to determine where to put them to minimize the overall probability that all four locations will fail simultaneously with the constraint that each location can assign not more than THREE backup drives. The estimated probabilities of failure are summarized in the following table.