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UPSC Prelims 2023CSATQuantitative AptitudeNumber System - Divisibility Rules

Q27. For any choices of values of X, Y and Z, the 6 digit number of the form XYZXYZ is divisible by:

A. 7 and 11 only
B. 11 and 13 only
C. 7 and 13 only
D. 7, 11 and 13✓ Correct

Detailed Solution

✓ Correct Answer: Option D

A 6-digit number of the form XYZXYZ can be expressed mathematically as: XYZXYZ = XYZ × 1000 + XYZ = XYZ × (1000 + 1) = XYZ × 1001 Now, we need to find the prime factors of 1001. 1001 can be divided by 7: 1001 ÷ 7 = 143.

143 can be divided by 11: 143 ÷ 11 = 13. 13 is a prime number. So, the prime factorization of 1001 is 7 × 11 × 13. Therefore, any number of the form XYZXYZ is always divisible by 7, 11, and 13.

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