Question 946814
the general formula for these types of problem is r * p * t = q


r = rate of work per person
p = number of persons
t = time 
q = quantity of work performed.


in this problem, you get:


r = what you want to solve for
p = 7
t = 40 minutes
q = 6 walls


formula becomes r * 7 * 40 = 6
solve for r to get r = 6 / 280


now that you know r, you can solvde for t in the second part of the problem.


you get r = 6/280
p = 4
t = what you want to solve for
q = 3


formula becomes 6/280 * 4 * t = 3
solve for t to get t = (3 * 280) / (4 * 6) = 35


your solution is that t = 35 minutes.


r * p * t = 3 becomes 6/280 * 4 * 35 = 3


simplify to get 3 = 3, confirming the solution is correct.