Business hours of public telephones: open at 9:00 am and close at 8:00 pm. It's only 8:40 in the morning. How many minutes do I have to wait before opening the door?
公用电话的营业时间：早9：00开门，晚8：00关门．现在才早上8：40，我还要等多少分钟才开门．
<result>20</result>