posted by Marta .
A contractor finds that it takes a crew A 6 hours to construct a wall of a certain size. Crew B takes 8 hours to construct a wall of the same size. How long will it take if they work together?
rate of A crew = wall/6
rate of B crew = wall/8
combined rate = wall/6 + wall/8
time with combined rate = wall/(7wall/24)
= 3.428 hours
= 3hours, 25.7 minutes
(notice the effective calculation was 1 ÷ (1/6 + 1/8)