q: A pipe can fill a cistern in 18 minutes whereas the cistern when fill can be emptied by a leak in 20 minutes. When both pipes are opened, find when the cistern will be full?
Options:
(A) 17
(B) 180
(C) 70
(D) 120
(E) 12
a: Stream of consciousness: 1/18 - 1/20 = 1/180
180 minutes The answer is (B).
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q: The average of 13 numbers is 60. Average of the first 7 of them is 57 and that of the last 7 is 61. Find the 8th number?
Options:
(A) 46
(B) 82
(C) 45
(D) 43
(E) 53
a: Stream of consciousness: Sum of all the 13 numbers = 13 * 60 = 780
Sum of the first 7 of them = 7 * 57 = 399
Sum of the last 7 of them = 7 * 61 = 427
So, the 8th number = 427 + 399 - 780 = 46. The answer is (A).
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q: Sarah's grade was 50th percentile of the 20 grades for her class. Of the 30 grades from another class, 10 were higher than Sarah's, and the rest were lower. If no other grade was the same as Sarah's grade, then Sarah's grade was what percentile of the grades of the two classes of two classes combined?
Options:
(A) 52nd
(B) 56th
(C) 60th
(D) 70th
(E) 82nd
a:

Show your work and conclude with the exact phrasing "Therefore, the final answer is [answer]." where [answer] is just the final number or expression that solves the problem.
user
Stream of consciousness: 52nd/56th/60th/70th/82nd The answer is (C).
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