Question 747868
Pro can do 1/16 return per hour
Novice can do 1/23 return per hour. 

Together, 1/16+1/23 = .0625 + .0435 = .10598 returns per hour or the reciprocal, 9.436 hours for one return.

This is choice D.

With no math, answers A,B, and C make no sense. If both were fast, it would be 8 hours, both slow, 11.5 hours. D is the only answer that is between these two extremes.