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GATE 2018 ME (ME1) – Question 5

General Aptitude · Quantitative Aptitude: Arithmetic and Number Computation · 1 mark · Multiple choice

A two-digit number has digit sum 9. Subtracting 45 interchanges its digits. What is the number?

  1. 63
  2. 72
  3. 81
  4. 90

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Correct answer: (B) 72

Explanation

Let digits be a and b. Then $a+b=9$ and $10a+b-45=10b+a$, giving $a-b=5$. Thus a = 7, b = 2, and the number is 72 (B).