QUESTION

CSAT

Medium

Maths

Prelims 2026

AA is a 2-digit number with different digits. BB is also a 2-digit number and is obtained by reversing the digits of AA. If ABA - B is a multiple of 2727, where A>BA > B, how many such different AA's are possible?

Select an option to attempt

Explanation

Answer: B — 99

Let the two-digit number AA be 10a+b10a+b, where aa and bb are digits and $a

eq b$.

Since BB is obtained by reversing the digits of AA, we have:

B=10b+aB=10b+a

Since BB is also a two-digit number, $b

eq 0$.

Now,

AB=(10a+b)(10b+a)=9a9b=9(ab)A-B=(10a+b)-(10b+a)=9a-9b=9(a-b)

It is given that ABA-B is a multiple of 2727. Therefore:

9(ab) is a multiple of 279(a-b) \text{ is a multiple of } 27

So, aba-b must be a positive multiple of 33.

Since A>BA>B, we have a>ba>b. The possible values of aba-b are:

3,6,93, 6, 9

Now count the valid digit pairs with a>ba>b and $b

eq 0$:

For ab=3a-b=3:

(a,b)=(4,1),(5,2),(6,3),(7,4),(8,5),(9,6)(a,b)=(4,1),(5,2),(6,3),(7,4),(8,5),(9,6)

This gives 66 numbers.

For ab=6a-b=6:

(a,b)=(7,1),(8,2),(9,3)(a,b)=(7,1),(8,2),(9,3)

This gives 33 numbers.

For ab=9a-b=9, the only possible pair is (9,0)(9,0), but this is not allowed because BB would not be a two-digit number.

Hence, the total number of possible values of AA is:

6+3=96+3=9

Therefore, the correct answer is 99.

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