JUstin and Hannah Both get jobs at McBurgers. Justin started on Sunday and gets every 9th day off. Hannah started the very next day and gets every 6 th day off. Justin would like to ask Hannah out on a date when they both have the day off from work. How many days will Justin have to wait before he can schedule his date
Justin starts on a Sunday, and gets every ninth day off.
So his days off, counting from Sunday, are always divisible by 9, therefore divisible by 3.
Hannah gets a day off every sixth day from Monday, i.e. every 6n+1 days from Sunday, or never divisible by 3.
Too bad for Justin, he will have to ask her out after work!
(See how mathematics can be important for social relations!)