No. 1 train departs every 6 minutes, No. 3 train departs every 15 minutes, after the two routes depart at the same time, at least how many minutes later the two routes depart at the same time.
1路车每6分钟发一班，3路车每15分钟发一班，两路车同时发出一班后，至少再过多少分钟两路车又同时发车．
<gadget id="calculator">2 * 3 * 5</gadget>
<output>30</output>

<result>30</result>