The concept that can be used in order to determine the answer to this item is that of equating the man-hours for each walls.
Case 1: (36 minutes)(7people) / 4 walls
Simplifying,
63 people-minutes/wall
Using this calculated value for the next case,
63 people-minutes/wall = (9 people)(x minutes)/7 walls
The value of x from the equation is 49 minutes.
Thus, the answer is 49 minutes.