he HAL Corporation wishes to improve the resistance of its personal computer to disk-drive

and keyboard failures. At present, the design of the computer is such that disk-drive failures

occur only one-third as often as keyboard failures. The probability of simultaneous disk-drive

and keyboard failures is 0.05. If the computer is 80 percent resistant to disk-drive and/or key-

board failure, how low must the disk-drive failure probability be?