Li Li goes to the library every three days and Wang Fang goes to the library every four days. They all went to the library on June 30th. When will they go at the same time next time? (The standard answer is: July 20th)

I understand it this way: every three days = every four days; Every four days = every five days.

That is, find the least common multiple of 4 and 5, 20.