A can construct a wall in 40 min and B can construct the wall in 45 min. How many hours is needed to contruct a wall if both the person working together. Choose the correct choice from the following answers.
A) 20 min
B) 22 min
C) 23 min
D) 21 min
E) 20 min
A's one minute work = 1/40   B's one minute work = 1/45   (A+B)'s one minute work = 1/40 + 1/45 = 85/40*45 = 17/360   so, (A+B)will do work together in 360/17 min = 21 3/17 minutes   21 min approximately   ANSWER:21 min
<result>D</result>
A can construct a wall in 40 min and B can construct the wall in 45 min. How many hours is needed to contruct a wall if both the person working together.