Numbers

Question:
Which one of the following can’t be the square of natural number ?

[A].

30976

[B].

75625

[C].

28561

[D].

143642

Answer: Option D

Explanation:

The square of a natural number nerver ends in 2.

143642 is not the square of natural number.

Question:
If a and b are odd numbers, then which of the following is even ?

[A].

a + b

[B].

a + b + 1

[C].

ab

[D].

ab + 2

Answer: Option A

Explanation:

The sum of two odd number is even. So, a + b is even.

Question: The smallest 3 digit prime number is:
[A].

101

[B].

103

[C].

109

[D].

113

Answer: Option A

Explanation:

The smallest 3-digit number is 100, which is divisible by 2.

100 is not a prime number.

101 < 11 and 101 is not divisible by any of the prime numbers 2, 3, 5, 7, 11.

101 is a prime number.

Hence 101 is the smallest 3-digit prime number.

Question:
Which one of the following is a prime number ?

[A].

161

[B].

221

[C].

373

[D].

437

Answer: Option C

Explanation:

437 > 22

All prime numbers less than 22 are : 2, 3, 5, 7, 11, 13, 17, 19.

161 is divisible by 7, and 221 is divisible by 13.

373 is not divisible by any of the above prime numbers.

373 is prime.

Question: Which one of the following is not a prime number?
[A].

31

[B].

61

[C].

71

[D].

91

Answer: Option D

Explanation:

91 is divisible by 7. So, it is not a prime number.

Question:
Which of the following is a prime number ?

[A].

33

[B].

81

[C].

93

[D].

97

Answer: Option D

Explanation:

Clearly, 97 is a prime number.

Question:
How many prime numbers are less than 50 ?

[A].

16

[B].

15

[C].

14

[D].

18

Answer: Option B

Explanation:

Prime numbers less than 50 are:
2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47

Their number is 15

Question: The sum of first five prime numbers is:
[A].

11

[B].

18

[C].

26

[D].

28

Answer: Option D

Explanation:

Required sum = (2 + 3 + 5 + 7 + 11) = 28.

Note: 1 is not a prime number.

Definition: A prime number (or a prime) is a natural number that has exactly two distinct natural number divisors: 1 and itself.

Question:
The smallest prime number is:

[A].

1

[B].

2

[C].

3

[D].

4

Answer: Option B

Explanation:

The smallest prime number is 2.

Question:
What is the unit digit in(795 – 358)?

[A].

0

[B].

4

[C].

6

[D].

7

Answer: Option B

Explanation:

Unit digit in 795 = Unit digit in [(74)23 x 73]

= Unit digit in [(Unit digit in(2401))23 x (343)]

= Unit digit in (123 x 343)

= Unit digit in (343)

= 3

Unit digit in 358 = Unit digit in [(34)14 x 32]

= Unit digit in [Unit digit in (81)14 x 32]

= Unit digit in [(1)14 x 32]

= Unit digit in (1 x 9)

= Unit digit in (9)

= 9

Unit digit in (795 – 358) = Unit digit in (343 – 9) = Unit digit in (334) = 4.

So, Option B is the answer.