I just copied and pasted your question on google and a graph came up it may not be the answer but i tried...
Okay so you're trying to find the unit rate of 720yds/4min. If you divide 720 by 4 you will have the unit rate as 180yds/1 min or 60 sec then if you divide 180 and 60 by 4 again you will get the answer of 45yds/15sec.
18 buses would depart in an hour.
Hope this helps :)