QUESTION

CSAT

Easy

Reasoning

Prelims 2017

A 2-digit number is reversed. The larger of the two numbers is divided by the smaller one. What is the largest possible remainder?

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Explanation

Let the original number be 10a+b10a + b, where aa is the tens digit and bb is the ones digit. Reversed number = 10b+a10b + a. Assume 10a+b>10b+a10a + b > 10b + a, so the larger number is 10a+b10a + b.

The remainder when dividing the larger number by the smaller one is:

(10a+b)÷(10b+a)(10a + b) \div (10b + a)

To maximize the remainder, set a=9a = 9 (largest possible tens digit). Original number = 90+b90 + b Reversed number = 10b+910b + 9

Now, test different values of bb:

  1. If b=4b = 4:
  • Original number = 9494
  • Reversed number = 4949
  • Division: 94÷4994 \div 49 → Quotient = 11, Remainder = 9449=4594 - 49 = 45
  1. If b=5b = 5:
  • Original number = 9595
  • Reversed number = 5959
  • Division: 95÷5995 \div 59 → Quotient = 11, Remainder = 9559=3695 - 59 = 36
  1. If b=6b = 6:
  • Original number = 9696
  • Reversed number = 6969
  • Division: 96÷6996 \div 69 → Quotient = 11, Remainder = 9669=2796 - 69 = 27
  1. If b=7b = 7:
  • Original number = 9797
  • Reversed number = 7979
  • Division: 97÷7997 \div 79 → Quotient = 11, Remainder = 9779=1897 - 79 = 18
  1. If b=8b = 8:
  • Original number = 9898
  • Reversed number = 8989
  • Division: 98÷8998 \div 89 → Quotient = 11, Remainder = 9889=998 - 89 = 9

The largest remainder is 4545, which occurs when the original number is 9494 and the reversed number is 4949.

Thus, the largest possible remainder is 4545.

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