Question 601601
    Alf can dig a trench in two hours, Bert takes three hours, Charlie takes 5 hours and Dave takes six hours. How long will they collectively take to dig the same trench if they all work simultaneously at their own speeds?
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Let x = time (hours) it takes collectively
then
x(1/2 + 1/3 + 1/5 + 1/6) = 1
multiply both sides by common denominator 30:
x(15 + 10 + 6 + 5) = 30
x(36) = 30
x = 30/36
x = 0.8333 hours
or 
x = 50 minutes