A certain number of answer sheets can be checked in 12 days working 5 hours each day by 9 examiners. For how many hours a day should 4 examiners work to check twice the number of answer sheets in 30 days ?

A. 4 hours

B. 8 hours

C. 7 hours

D. 9 hours

Answer : D. 9 hours

Explanation:

Let required number of hours be x
More examiners, less hours(indirect proportion)
More days, less hours(indirect proportion)
More answer books, more hours(direct proportion)
Hence we can write as

A certain number of answer sheets
    • 9 * 12 * 2 * 5 = 4 * 30 * 1 * x
    • x = 9 * 12 * 2 * 5 / 4 * 30 * 1
    • = 9 * 3 * 2 * 5 / 30
    = 9

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