This is the problem of examining the least common multiple. A goes to the library every five days, which means A goes to the library every six days. According to this principle, B walks every 12 days, C walks every 18 days, and D walks every 30 days.

Then the lowest common multiple of 6, 12, 18,30 is 180, that is, 180 days later, and now it is18 in May, 180 days later.

Answer over!