9 examiners can examine a certain number of answer books in 12 days by working 5 hours a day. How many hours in a day should 4 examiners work to examine twice the number of answer books in 30 days?	Choose the correct choice from the following options:
A) 9  B) 10  C) 11  D) 12  E) 15
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   (examiners) 9:4   (days) 12:30   (answer books) 2:1 }::x:5   ⇒9×12×2×5=4×30×1×x   ⇒x=9×12×2×5/4×30×1   =9×3×2×5/30   =9×2×5/10   =9   9nswer: Option 9
<result>A</result>
9 examiners can examine a certain number of answer books in 12 days by working 5 hours a day. How many hours in a day should 4 examiners work to examine twice the number of answer books in 30 days?