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555-605 (Medium)|   Arithmetic|                           
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Bunuel
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This is a good question, a tricky one, can take above 5 mins or even less than 2 mins.... So let's see a different approach

it's sum of 2 two digit integers: AB, BA i.e. digits have been interchanged

AB = 10A + B
BA = 10B + A

Sum = 11A + 11B .i.e. 11(A+B)

question is asking units digit of AAC.

From above we can say AAC is a multiple of 11, with Tens & Hundred digits same. so we can have 110, 220,330,........ in any case number will not be above 110 (since we are summing 2 two digit integers)
Hence C is 0.

ANSWER (E).

Just for a note this question can be turned into a good DS question too. This is the beauty of this question.
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It can also be solved as follows:
AB = 10*A+B
BA = 10*B+A
AAC = 100*A+10*A+C

So, (10A+B)+(10B+A)=(100A+10A+C)
11(A+B)=110A+C
A+B=10A+(C/11)
As A,B,C must be Integers (since basically they are digits of numbers) hence C must be multiple of 11. Now C must be Single Digit and only single digit multiple of 11 is 0.
Hence, C=0
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I have an algebraic approach to it:

AB + BA = (10*A + B) + (10*B + A) = 11 (A+B)

AAC = 100*A + 10*A + C = 110*A + C

Given, AB + BA = AAC
=> 11A + 11B = 110A + C
=> C = 11B - 99A
=> C = 11(B -9A)
Since A & B are positive integers, they have to be to 9. While doesn't have to be positive, it cannot be negative. Hence C can be 0 to 9.
With A & B both positive, only way to have non-negative C is to have B - 9A >=0. A = 1 & B = 9 is the only solution to it.
Hence, 19 + 91 = 110, hence C = 0

This is my 1st answer guys, if there is anything I am missing out, please excuse!
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AB + BA = AAC
Then 10A+B+10B+A= 11(A+B)

AAC has to be multiple of 11 such that 10s and 100s digit is the same.
Therefore possible answers are 110, 220.....
110=11*10
220=11*20

Since A and B is single right then A+B max value is 9+8=17 since A and B distinct integer!!!
Now only possible value of A+B is 10 (since it is less than 17)
So Unit Digit is 0
Answer E

Posted from my mobile device
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Its like AB+BA ==AAC

AB = 10a + b
BA = 10b + a
+ = 11(a+b)

Therefore AAC = 11(a+b),

Now AAC is 3 digit multiple of 11 and also 100th and 10th digit are same and we know minimum of it will be 110 (1-9 multiple of 11 are two digits)

Therefore unit digit is 0.
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AB + BA = (10x+y) + (10y+x) = 11(x+y)

So, the sum is a multiple of 11. Divisibility of 11 is checked by adding the first set of alternate numbers then subtracting from it sum of next set of the alternate numbers, if the result is 11 or 0 then the no. is divisible by 11 (e.g., to check if 132 is divisible by 11, we calculate 1+2-3 which is 0 hence it’s divisible by 11)

In the given question, the sum is AAC and A+C-A=C should be either 11 or 0. Since 11 isn’t a digit, C can’t be 11 and must be 0.
Answer is E
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singh_amit19
AB
+BA
-----------
AAC

In the correctly worked addition problem shown, where the sum of the two-digit positive integers AB and BA is the three-digit integer AAC, and A, B, and C are different digits, what is the units digit of the integer AAC?

A. 9
B. 6
C. 3
D. 2
E. 0





Nick Slavkovich, GMAT/GRE tutor with 20+ years of experience

[email protected]
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