Important number system math question and answer for bank,bcs and others compititive exams part-1

 Important number system math question and answer for bank,bcs and others compititive exams

 Prime number related question and Answer

 Question: Which of the following is a prime number

A. 19 

B. 20

C. 21

D. 22

Answer: A

Solution: Clearly,19 is a prime number.

Question: Which is not a prime number?

A. 13

B. 19

C. 21

D. 17

Answer: C

Solution: 21=3*7 is not a prime number because 21 is a composite number.

Question: which of the following is a prime number?

A. 143

B. 289

C. 117

D. 359

Answer: D

Solution: Clearly,143=13*11 so, 143 is not prime. 289=17*17 so ,289 is not a prime  number. 359 is not exactly divisible by any of them. so 359 is a prime number.

Question: The smallest value of natural number n, for which 2n+1 is not a prime number, is

A. 3

B. 4

C. 5

D. None

Answer B

Solution: Putting n=1,2,3,4 respectively in (2n+1)

we get: (2*1+1)=1, (2*2+1)=5, (2*3+1)=7

and (2*4+1)= 9, Where 3,5,7 are prime numbers. But 9 is not a prime number.

The smallest value of n for which (2n+1) is not a prime , is n=4.

Factors related queations and answer

Question: P and Q are two positive integers such that PQ=64. Which of the following cannot be the value of P+Q?

A. 16

B. 20

C. 35

D.65

Answer: C

Solution: We may have (64 and 1), (32 and 2), (16 and 4), and (8 and 8). in any case , the sum is not 35.

Question: The number if prime factors in the expression 6^10. 7^17. 11^27 is equal to.

A. 54

B. 64

C. 71

D. 81

Answer: B

6^10. 7^17. 11^27=(2*3)^10*7^17*11^27= 2^10*3^10*7^17*11^27

Number of Prime factors in the given expression=(10+10+17+27)= 64