A, B, C, D can complete a work in 3, 6, 9, 12 hours respectively. Further, only one person can work at a time in each hour and nobody can work for two consecutive hours. It is not necessary to engage all. What is the minimum number of hours that they will take to finish the work ?
A.36/25
B.12/5
C.4✓ Correct
D.2
Explanation
Rates: A=1/3, B=1/6, C=1/9, D=1/12 per hour. Since no one can work consecutively, we alternate. To minimize time, use fastest workers alternately. Hour 1: A does 1/3. Hour 2: someone else (B does 1/6). Hour 3: A does 1/3. Hour 4: B does 1/6. Total after 4 hours = 2(1/3) + 2(1/6) = 2/3 + 1/3 = 1. So work finishes in 4 hours.
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