Question 175440
times to unload a certain truck: 
al,bob,chuck together t hours
al alone t+1
bob alone t+6
chuck alone 2t 
find t
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You are given the time for each person and the group.
For example Al alone takes (t+1) hr/job
Invert that to get Al's rate: 1/(t+1) job/hr
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Equation:
rate + rate + rate = together rate
1/(t+1) + 1/(t+6) + 1/2t = 1/t

Multiply thru by 2t(t+1)(t+6) to get:

2t(t+6) + 2t(t+1) + (t+1)(t+6) = 2(t+1)(t+6)

2t^2+12t + 2t^2 + 2t + t^2 + 7t + 6 = 2t^2 + 14t + 12

3t^2 + 7t -6 = 0

3t^2 + 9t - 2t-6 = 0
3t(t+3) -2(t+3) = 0
(t+3)(3t-2) = 0
Positive solution:
t = 2/3 hr
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Cheers,
Stan H.