There are two old wall clocks in Xiaoming's house, one is 20 minutes faster every day, and the other is 30 minutes slower every day. Now, if these two old wall clocks are set to standard time at the same time, how many days will it take for them to display standard time at the same time again?
小明家有两个旧挂钟，一个每天快20分，一个每天慢30分．现在将这两个旧挂钟同时调到标准时间，它们至少要经过多少天才能再次同时显示标准时间？
<result>72</result>