Answer: 13 buses.
Explanation:
When a Northbound bus leaves Waco there are 7 buses on the way from Dallas, including the one that is arriving at Waco and one departing from Dallas.
If you do not visualize it, think that the Northbound bus departs at 6:00 am, then the bus arriving at Waco departed at 5:00 am, other departed at 5:10am, other at 5:20, other at 5:30, other at 5:40, other at 5:50 and other at 6:00 am. Those are 6 buses that the Northbound bus will encounter on its way.
After that, there will departure other 6 buses that the Nortbound bus will encounter. The times for those departures are: 6:10am, 6:20am, 6:30am; 6:40am, 6:50am and, the last one, 7:00am just when it is arriving.
Therefore, the Northbound bus will encounter 7 buses + 6 buses = 13 buses, which is the answer.
