Calculate the date of their fifth meeting in the library. When did they meet in the library for the fourth time?
Solution: A goes every other day, once every two days, and B goes every five days, once every six days. The least common multiple of 2 and 6 is 6, that is, they will meet once every six days and on the first day of every six days, so they will meet once in July 1 day, and it takes three six days plus four six days to meet again.