Central Exam — Quantitative Aptitude
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Q.131 Medium Numbers
If the sum of two consecutive odd numbers is 56, what is the smaller number?
A 25
B 26
C 27
D 28
Correct Answer:  C. 27
Explanation:

Let smaller odd number = x.

Then x+(x+2)=56.

So 2x+2=56, 2x=54, x=27.

Check: 27+29=56 ✓

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Q.132 Hard Numbers
A number has exactly 3 factors. Which of the following must be true?
A It is a perfect square
B It is the square of a prime number
C It is divisible by 3
D It is an even number
Correct Answer:  B. It is the square of a prime number
Explanation:

A number has exactly 3 factors only when it is the square of a prime.

For p² where p is prime, factors are: 1, p, p².

Example: 4 has factors 1,2,4 (3 factors). 9 has factors 1,3,9 (3 factors).

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Q.133 Hard Numbers
What is the sum of all divisors of 28?
A 56
B 64
C 72
D 84
Correct Answer:  A. 56
Explanation:

Divisors of 28: 1, 2, 4, 7, 14, 28.

Sum = 1+2+4+7+14+28 = 56. (Note: 28 is a perfect number where sum of proper divisors = 28)

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Q.134 Hard Numbers
If a number is expressed as 2³×3²×5, what is the total number of divisors?
A 12
B 24
C 30
D 36
Correct Answer:  B. 24
Explanation:

For n = p₁^a × p₂^b × p₃^c, number of divisors = (a+1)(b+1)(c+1).

Here: (3+1)(2+1)(1+1) = 4×3×2 = 24

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Q.135 Easy HCF and LCM
Find the HCF of 48 and 64.
A 8
B 16
C 32
D 24
Correct Answer:  B. 16
Explanation:

Using prime factorization: 48 = 2⁴ × 3, 64 = 2⁶. HCF = 2⁴ = 16 (taking lowest powers of common prime factors).

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Q.136 Easy HCF and LCM
What is the LCM of 12 and 18?
A 36
B 24
C 54
D 72
Correct Answer:  A. 36
Explanation:

Prime factorization: 12 = 2² × 3, 18 = 2 × 3². LCM = 2² × 3² = 4 × 9 = 36 (taking highest powers of all prime factors).

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Q.137 Easy HCF and LCM
Find the HCF of 56 and 72.
A 4
B 8
C 12
D 16
Correct Answer:  B. 8
Explanation:

This question asks us to find the Highest Common Factor (HCF) of two numbers using prime factorization or the Euclidean algorithm.

Step 1: Prime factorization of 56

Express 56 as a product of prime numbers.

\[56 = 2 \times 2 \times 2 \times 7 = 2^3 \times 7\]
Step 2: Prime factorization of 72

Express 72 as a product of prime numbers.

\[72 = 2 \times 2 \times 2 \times 3 \times 3 = 2^3 \times 3^2\]
Step 3: Find the HCF

The HCF is the product of common prime factors with their lowest powers.

\[\text{HCF} = 2^3 = 8\]

The HCF of 56 and 72 is 8, making the correct answer (B).

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Q.138 Easy HCF and LCM
The LCM of 15 and 25 is:
A 75
B 150
C 225
D 300
Correct Answer:  A. 75
Explanation:

This question asks us to find the Least Common Multiple (LCM) of two numbers using prime factorization.

Step 1: Find prime factorization of 15

Break 15 into its prime factors.

\[15 = 3 \times 5\]
Step 2: Find prime factorization of 25

Break 25 into its prime factors.

\[25 = 5 \times 5 = 5^2\]
Step 3: Calculate LCM using highest powers of all prime factors

The LCM is found by taking the highest power of each prime that appears in either factorization: 3¹ and 5².

\[\text{LCM} = 3^1 \times 5^2 = 3 \times 25 = 75\]

The LCM of 15 and 25 is 75.

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Q.139 Medium HCF and LCM
Find the HCF and LCM of 36 and 48. What is their product?
A 1728
B 1440
C 1296
D 1680
Correct Answer:  A. 1728
Explanation:

36 = 2² × 3², 48 = 2⁴ × 3. HCF = 2² × 3 = 12, LCM = 2⁴ × 3² = 144.

Product = 36 × 48 = HCF × LCM = 12 × 144 = 1728.

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Q.140 Medium HCF and LCM
Two numbers have HCF of 8 and LCM of 96. If one number is 24, find the other number.
A 32
B 40
C 48
D 64
Correct Answer:  A. 32
Explanation:

Using formula: HCF × LCM = Product of two numbers. 8 × 96 = 24 × x. 768 = 24x. x = 32.

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