The backup of the data stored in a computer is what allows a clean starting point, inside the machine, which allows the user to have a great security of the files stored by him. In that case, as the question above states, returning files to the computer will be easier than redoing everything manually. Back up is a security strategy, where the user copies all important files to a storage device, where the files can be recovered, in case the computer needs to be restored and this forces the deletion of the original files.
The least common multiple of 9 and 6 is 18 so in 18 months she will replace the bulb for the 2nd time and the air filter for the 3rd time. It will be the first time they are replaced together.
