A tour of the Museum and the Zoo begin at 8:30. Tour of the Museum leave every 15 minutes. Tours for the Zoo leave every 20 minutes. How frequently do the tours leave at the same time?
Museum tours: 8:30 , 8:45, 9:00, 9:15 , 9:30, ....
Zoo tours: 8:30, 8:50, 9:10, 9:30, ....