You need some specific information. Do you know how many decks the Deck Swab has to scrub? How many can he scrub in one hour? How many hours does he plan to scrub?
Think about how many decks he will scrub in 5 hours.
Even if the Deck Swab scrubs for 5 hours, there will still be decks left to scrub. How many decks will he finish in the whole time he scrubs? How many decks are left?