Rationale: 1/20 - 1/40 = 1/40
40 minutes
The answer: (C)
Question: A pipe can fill a cistern in 20 minutes whereas the cistern when fill can be emptied by a leak in 40 minutes. When both pipes are opened, find when the cistern will be full?
Options:
(A) 17 minutes
(B) 87 minutes
(C) 40 minutes
(D) 27 minutes
(E) 11 minutes

Rationale: Let number of boys = x
Let number of girls = y
Therefore x/y = B and y/x = G
10(B + G) = 10[(x/y) + (y/x)] = 10[(x2 + y2)/xy] > 10
The answer: (E)
Question: If the ratio of boys to girls in a class is B and the ratio of girls to boys is G; then 10 (B + G) is ?
Options:
(A) Equal to 3
(B) Less than 3
(C) More than 3
(D) Less than 1/3
(E) none of these

Rationale: 91 is divisible by 7. So, it is not a prime number.
The answer: (D)
Question: Which one of the following is not a prime number?
Options:
(A) 31
(B) 61
(C) 71
(D) 91
(E) 101