q: 10 books are placed at random in a shelf. The probability that a pair of books will always be together is?
Options:
(A) 1/6
(B) 1/7
(C) 1/5
(D) 1/9
(E) 1/2
a: OK... 10 books can be rearranged in 10! ways consider the two books taken as a pair then number of favourable ways of getting these two books together is 9! 2!
Required probability = 1/5
The answer is (C).

q: Fill in the blank with the correct number in this number series: 1, 20, 7, 16, __, 12, 19, ?
Options:
(A) 14
(B) 12
(C) 13
(D) 15
(E) 11
a: OK... This is a simple alternating subtraction and addition series; The first series begins with 1 and adds 6; the second begins with 20 and subtracts 4, and so on.
The answer is (C).

q: What will come in place of the x in the following Number series? 68, 36, 20, 12, 8, x
Options:
(A) 6
(B) 4
(C) 2
(D) 1
(E) 0
a:
OK... 68
68 / 2 + 2 = 36
36 / 2 + 2 = 20
20 / 2 + 2 = 12
12 / 2 + 2 = 8
8 / 2 + 2 = 6
The answer is (A).