silverflamein wrote:Craig, Mark and Sue complete a certain piece of work in 10,20 and 5 hours respectively. Craig works alone for the first 2 hours. Mark and Sue then join Craig and the three complete the work together. How long did it take to complete the task ?
A - 3hrs
B - 3 4/7hrs
C - 4 hrs
D - 4 2/7 hrs
E - 4 5/7 hrs
Let the job = 20 units.
Rate for Craig = w/t = 20/10 = 2 units per hour.
Rate for Mark = w/t = 20/20 = 1 unit per hour.
Rate for Sue = w/t = 20/5 = 4 units per hour.
Work produced by Craig in 2 hours = r*t = 2*2 = 4 units.
Remaining work = 20-4 = 16 units.
Combined rate for C+M+S = 2+1+4 = 7 units per hour.
Time to finish the job = w/r = 16/7 hours.
Total time = 2 + 16/7 = 30/7 hours.
The correct answer is
D.
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