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A, B, C, and D can individually complete a task

Question: A, B, C, and D can individually complete a task in 10, 15, 20, and 30 days respectively. How long will they take if all 4 work together?
A) 3.42 days
B) 4 days
C) 3.75 days
D) 4.25 days

Solution:

Work per day by A = 1⁄10, B = 1⁄15, C = 1⁄20, D = 1⁄30.
Total work per day = 1⁄10+1⁄15+1⁄20+1⁄30 = 6⁄60+4⁄60+3⁄60+2⁄60 = 15⁄60 = 1⁄4
Time to complete = 1/1⁄4 = 4 days.

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