Solution:
Since the hour hand is equal, the minute hand decides the time. Subtract 5:05 from 5:51 as 51 is greater than 05. Subtract 5 from 51 to obtain the time Tony was at the airport.
- 5:51 - 5:05
- ⇒ 51 - 5
- ⇒ 46 minutes
Tony was at the airport for 46 minutes.