There is a musical clock that rings at regular intervals. It is already known that the bell will sound at 8:00, 8:20, 8:40, 9:00, 9:20 and 9:40 in the morning, and how many minutes the bell will sound.
有一个音乐钟，每隔一段时间就发出铃声．已经知道上午8：00、8：20、8：40、9：00、9：20和9：40发出铃声，每隔多少分钟就发出铃声．
<result>20</result>