Puzzle 007: Cleaning Mops

ナゾ００７　モップでお掃除
１日１回使うと、最低２日間は干さな
ければならないモップがある。しかも、
このモップは３回使うと痛んでしまい
使えなくなる。
このモップを使って、１日に１回、
３０日間続けて掃除をするとしたら、
最低何本おモップが必要になるだろう？

Google Translate
Times per day and use at least two days should have 干Sanakere mop. Moreover, this mop is not to be used three times away sorely use.
Mop with this one once a day, after 30 days in a row and do the cleaning, the mop will need something from your book?

Notes on Puzzle
A tricky logic puzzle, asking you how many mops you need to use if you want to clean continuously for 30 days (these are some pretty crappy mops. I mean, it takes two days to dry out and each time you use them they start breaking down? Yeesh. That's what a Swiffer is for!) I "solved" it by going forwards and backwards from my original number until I got the right answer. There's a fairly good leap in logic that I didn't get. I'm not sure whether it was just me being a dunce, or me not fully understanding the conditions in Japanese.