A system administrator notices that a file service disk is failing for two unrelated reasons. Once every 30 days, on average, vibration due to nearby construction breaks the disk's arm. Once every 60 days, on average, a power surge destroys the disk's electronics. The system administrator fixes the disk instantly each time it fails. The two failure modes are independent of each other, and independent of the age of the disk. What is the mean time to failure of the disk?