At the flower show, one trolley arrives every 4 minutes to take people to the rose pavilion. Another trolley arrives every 10 minutes to take people to the exotic flower greenhouse. Both trollies just arrived at the main building. How many minutes from now will be the next time the trollies arrive at the main building together?

The rose pavilion trolley will arrive in another four minutes, then eight, then twelve, sixteen, twenty. The exotic flower greenhouse trolley will arrive in another ten minutes, then twenty. Four doesn't go into ten, but four and ten both go into twenty. The next two trollies will arrive at the main building together in twenty minutes.