Anthony and Billy can built a wall in 6 days, Billy and Corey can do it in 9 days and Corey and Anthony in 12 days. How many days will it take the three of them together to build the wall?
+36916 Let's see how long it wil take them to build THREE walls....then divide the resulting time by 3
1/6 + 1/9 + 1/12 = 3/x
(12 + 8 + 6) / 72 = 3/x
26/72 = 3/x
x = 3 * 72/26 x = 8.30769 for THREE walls
One wall = 2.7692 days
EP, your solution method will not give the correct result. 
The performance times (contributions) of an individual worker is combined (buried) with the times of two (2) other individual workers. These individual times have to be extracted (separated) from the data before they are recombined for a collective result.
Note that by doubling your result, the time will then be very close to the correct calculated time. This is because the times used in your calculation are based on two workers.
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+36916 Yah....you are CORRECT.....had been thinking a bit about that....another answer soon..... bummer ! HaHa
+36916 *** UPDATE *** My answer above is incorrect....
1/(A+B) = 6 1/(B+C) = 9 1/(C+A) = 12
B = (1-6A)/6 C = (1-12A)/12
1/ [ (1-6A)/6 + (1-12A)/12]
Solve for A = 15/216
then B= 21/216
and C = 4/216
1/(A+B+C) = x
1/(40/216) = x
216/40 = x = 5.4 days (IF I did all of the calcs w/out error!!)
