## Solution
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
## What the question and answer could be
10 books are placed at random in a shelf. The probability that a pair of books will always be together is -
Options:
(A) 1/5
(B) 1/3
(C) 1/4
(D) 118/2
(E) 21/4
(C)

## Solution
Number = (12 * 35) = 420
Correct quotient = 420/21 = 20
## What the question and answer could be
In a division sum, the remainder is 0. As student mistook the divisor by 12 instead of 21 and obtained 35 as quotient. What is the correct quotient?
Options:
(A) 0
(B) 12
(C) 13
(D) 20
(E) 22
(D)

## Solution
Speed downstream = d/t = 90/(2 1/2) = 36 kmph
Speed upstream = d/t = 45/(2 1/2) = 18 kmph
The speed of the stream = (36 - 18)/2 = 9 kmph
## What the question and answer could be
A man rows his boat 90 km downstream and 45 km upstream, taking 2 1/2 hours each time. Find the speed of the stream?
Options:
(A) 5 kmph
(B) 7 kmph
(C) 9 kmph
(D) 8 kmph
(E) 1 kmph
(C)

## Solution
P = (P*40*R)/100
R = 2 1/2%
## What the question and answer could be
At what rate percent on simple interest will a sum of money double itself in 40 years?
Options:
(A) 3 1/3%
(B) 2 1/2%
(C) 3 9/3%
(D) 3 2/3%
(E) 3 6/3%
(B)