Which of the following is a pair of co-prime numbers? 

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RRB Technician Grade III Official Paper (Held On: 30 Dec, 2024 Shift 2)
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  1. 52, 24 
  2. 81, 16 
  3. 455, 49 
  4. 363, 77 

Answer (Detailed Solution Below)

Option 2 : 81, 16 
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Detailed Solution

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Formula used:

Co-prime numbers are pairs of numbers that have no common divisor other than 1.

Calculation:

Option 1: 52 and 24

Prime factors of 52 = 2 × 2 × 13

Prime factors of 24 = 2 × 2 × 2 × 3

Common factors: 2

⇒ Not co-prime

Option 2: 81 and 16

Prime factors of 81 = 3 × 3 × 3 × 3

Prime factors of 16 = 2 × 2 × 2 × 2

Common factors: None

⇒ Co-prime

Option 3: 455 and 49

Prime factors of 455 = 5 × 7 × 13

Prime factors of 49 = 7 × 7

Common factors: 7

⇒ Not co-prime

Option 4: 363 and 77

Prime factors of 363: 3 × 11 × 11

Prime factors of 77: 7 × 11

Common factors: 11

⇒ Not co-prime

∴ The correct answer is option 2.

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