If you're asking from academic interest, you'd need to consider whether striping across disks increases or decreases the likelihood of failure due to different read/write behaviour. (I don't know what the contributing factors to a disk failing are - i.e. is it purely random, actively increased by use, etc.) That formula doesn't include any such parameters; it may well be an approximation of something that does. I don't see on the linked Wikipedia page where the poster apparently sourced it from, so it's currently just a way to produce a number slightly less than directly multiplying the failure rate by number of drives.