Think of it in terms of per unit of time, or the rate.
Person A can dig a hole in 5 hours, so he can do 1/5 of a hole per hour.
Person B can dig a hole in 6 hours, so he can do 1/6 of a hole per hour.
That means together they can dig a hole 1/5 + 1/6 = 11/30 of a hole per hour.
That means together they can dig a hole in 30/11 hours.
Or 2 and 8/11 of an hour. Or roughly 2 hours and 44 minutes.
