Question 323473
It's a matter of perspective.
Ryan digs the hole in 5 hours, which is 1/5 hole per hour.
Castel digs it in 6 hours, which is 1/6 hole per hour.
Together, they do the sum, 1/5 + 1/6 per hour = 11/30 holes per hour.
11/30 hole/hour --> 30/11 hours/hole
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Use the shortcut, product/sum
5*6/(5+6) = 30/11