Problem

A multi-user mainframe computer includes two disk drives that are prone to failure. If a disk drive fails, users lose their files, which need to be restored. Restoration is achieved by copying copies of the files, held on magnetic tapes, onto the disk backup. This restoration is inconvenient and so a new operation policy is being considered. At the moment, the disk drives are repaired and restored as and when they fail. The proposal is to introduce a joint repair system. The life of a disk drive is Normally distributed with a mean of 4 months and a standard deviation of 1 month. Under the current policy, it costs $50 to repair and restore a failed disk drive. The joint repair and restore policy would operate as follows: when either unit fails, the failed unit is repaired and restored, and the other unit is cleaned to bring it to a state equivalent to having been just repaired. This will cost $75 in total. Is the new joint repair and restore policy cost effective? Use Excel to simulate 200 individual failures (100 for each disk) and 100 joint repairs. ( how do we tackle this problem)