CAT Practice : Number System

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How scientifically can you do trial-and-error for this question?

Perfect Square

    Q.3: A 4-digit number of the form aabb is a perfect square. What is the value of a - b?
    1. 3
    2. 2
    3. 4
    4. 1


  • Correct Answer
    Choice A. 3

Detailed Solution

A number of the form aabb has to be a multiple of 11. So, it is the square of either 11 or 22 or 33 on up to 99.

882 = 7744.

This is the only solution possible. Most of these trial and error questions need to be narrowed down a little bit before we can look for the solution. That narrowing down is critical. In this case, we should look for multiples of 11.

Correct Answer: 3

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More questions from Number System

  1. Product of digits
  2. Sum of first n natural numbers
  3. Perfect Square
  4. Last two digits
  5. Multiples
  6. Greatest integer function
  7. Medians, Number
  8. 44th digit of a number
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