Question 515208
The problems hinge on the fraction of work each can do per day, times the number of days.
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(1/6 + 1/4 + 1/2)x = 1 whole job
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(2/12 + 3/12 + 6/12)x = 1
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11/12*x =1 
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11x = 12
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x = 12/11
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So working together they can do the job in 12/11 of a day.
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Done.