A and B represent digits

Question

A B + B ____ B A ____ In the correctly worked addition problem above, A and B represent digits. What is A? (A) 5 (B) 6 (C) 7 (D) 8 (E) 9

spartacus1412 · Answer

Answer is 8 10A +B +B = 10B +A a/b =8/9

Brent@GMATPrepNow · Answer

sanju09 wrote:A B
+ B
____
B A
____

In the correctly worked addition problem above, A and B represent digits. What is A?
(A) 5
(B) 6
(C) 7
(D) 8
(E) 9

We can apply some logic here.

We can make 2 conclusions:

1) When we add B+B the units digit of the sum is A
This means that A must be even. So, we can eliminate A, C and E.

2) When we add B+B, the sum is greater than 9.
We know this because we begin with a tens digit of A (as in AB) and once we add B, the sum becomes BA where the tens digit is now B.

Since we already know that the answer is B or D, we can just check each one.
This won't take long since we need only check one of these answer choices. If B works, we're done. If B doesn't work, the answer must be D.

Let's check answer choice B, which says A=6
If A = 6, then B must equal 8 (since 8+8=16)
If A=6 and B=8, we get: 68+8=86 (doesn't work, eliminate B)

So, the answer must be D

. . . Now, if we want to kill some time before the next question we COULD check this answer choice, but I'd just move on to the next question.

Let's check answer choice D (for the timid!

)

Answer choice D says A=8
If A = 8, then B must equal 9 (since 9+9=18)
If A=8 and B=9, we get: 89+9=98 (WORKS!)

So, the answer is D

Cheers,
Brent

Brent Hanneson - Creator of GMATPrepNow.com

deojason · Answer

For some reason, this question is very confusing to me.

How did you come up with your first statement that "When we add B+B the units digit of the sum is A
This means that A must be even. So, we can eliminate A, C and E."

I'm lost. Thanks!

Brent@GMATPrepNow · Answer

deojason wrote: How did you come up with your first statement that "When we add B+B the units digit of the sum is A
This means that A must be even. So, we can eliminate A, C and E."

Let's examine some sums of two numbers that have the same units digit.
23 + 3 = 26
51 + 1 = 52
87 + 7 = 94
14 + 4 = 18

Notice that the units digit of the sum is equal to the units digit in the sum of two identical units digits.

Cheers,
Brent

Brent Hanneson - Creator of GMATPrepNow.com

deojason · Answer

Awesome! Thanks a lot! That makes sense now. I feel funny I didn't catch it the first time. I think it's time to rest since I've been watching your videos on Youtube, reading blogs, and answering practice questions for at least 5 hours now. LOL

Thanks a lot! You're a great help!

Brent@GMATPrepNow · Answer

sanju09 wrote:A B
+ B
____
B A
____

In the correctly worked addition problem above, A and B represent digits. What is A?
(A) 5
(B) 6
(C) 7
(D) 8
(E) 9

We can apply some logic here.

We can make 2 conclusions:

1) When we add B+B the units digit of the sum is A
This means that A must be even. So, we can eliminate A, C and E.

2) When we add B+B, the sum is greater than 9.
We know this because we begin with a tens digit of A (as in AB) and once we add B, the sum becomes BA where the tens digit is now B.

Since we already know that the answer is B or D, we can just check each one.
This won't take long since we need only check one of these answer choices. If B works, we're done. If B doesn't work, the answer must be D.

Let's check answer choice B, which says A=6
If A = 6, then B must equal 8 (since 8+8=16)
If A=6 and B=8, we get: 68+8=86 (doesn't work, eliminate B)

So, the answer must be D

. . . Now, if we want to kill some time before the next question we COULD check this answer choice, but I'd just move on to the next question.

Let's check answer choice D (for the timid!

)

Answer choice D says A=8
If A = 8, then B must equal 9 (since 9+9=18)
If A=8 and B=9, we get: 89+9=98 (WORKS!)

So, the answer is D

Cheers,
Brent

Brent Hanneson - Creator of GMATPrepNow.com

regor60 · Answer

sanju09 wrote:A B
+ B
____
B A
____

In the correctly worked addition problem above, A and B represent digits. What is A?
(A) 5
(B) 6
(C) 7
(D) 8
(E) 9

https://www.crushthetest.com

As written AB + B = BA representing digits.

So 10A + 2B = 10B + A

Rearranging, 9A - 8B = 0

So 9A= 8B. Pretty easy to see the LCM is 72

therefore A=8, D and B = 9

Shamanthi · Answer

Brent@GMATPrepNow wrote:sanju09 wrote:A B + B ____ B A ____ In the correctly worked addition problem above, A and B represent digits. What is A? (A) 5 (B) 6 (C) 7 (D) 8 (E) 9 Alternatively, we can apply some logic here. We can make 2 conclusions: 1) When we add B+B the units digit of the sum is A This means that A must be even. So, we can eliminate A, C and E. 2) When we add B+B, the sum is greater than 9. We know this because we begin with a tens digit of A (as in AB) and once we add B, the sum becomes BA where the tens digit is now B. Since we already know that the answer is B or D, we can just check each one. This won't take long since we need only check one of these answer choices. If B works, we're done. If B doesn't work, the answer must be D. Let's check answer choice B, which says A=6 If A = 6, then B must equal 8 (since 8+8=16) If A=6 and B=8, we get: 68+8=86 (doesn't work, eliminate B) So, the answer must be D . . . Now, if we want to kill some time before the next question we could check this answer choice, but I'd just move on to the next question. Let's check answer choice D (for the timid ) Answer choice D says A=8 If A = 8, then B must equal 9 (since 9+9=18) If A=8 and B=9, we get: 89+9=98 (WORKS!) So, the answer is D Cheers, Brent Hi Brent, I think there is a typo in the analysis of the answer B. 68+8= 76 (not 86) making it unsuitable for the answer.

Brent@GMATPrepNow · Answer

Shamanthi wrote:Brent@GMATPrepNow wrote:sanju09 wrote:A B + B ____ B A ____ In the correctly worked addition problem above, A and B represent digits. What is A? (A) 5 (B) 6 (C) 7 (D) 8 (E) 9 Alternatively, we can apply some logic here. We can make 2 conclusions: 1) When we add B+B the units digit of the sum is A This means that A must be even. So, we can eliminate A, C and E. 2) When we add B+B, the sum is greater than 9. We know this because we begin with a tens digit of A (as in AB) and once we add B, the sum becomes BA where the tens digit is now B. Since we already know that the answer is B or D, we can just check each one. This won't take long since we need only check one of these answer choices. If B works, we're done. If B doesn't work, the answer must be D. Let's check answer choice B, which says A=6 If A = 6, then B must equal 8 (since 8+8=16) If A=6 and B=8, we get: 68+8=86 (doesn't work, eliminate B) So, the answer must be D . . . Now, if we want to kill some time before the next question we could check this answer choice, but I'd just move on to the next question. Let's check answer choice D (for the timid ) Answer choice D says A=8 If A = 8, then B must equal 9 (since 9+9=18) If A=8 and B=9, we get: 89+9=98 (WORKS!) So, the answer is D Cheers, Brent Hi Brent, I think there is a typo in the analysis of the answer B. 68+8= 76 (not 86) making it unsuitable for the answer. I'm not saying that 68+8=86 I'm saying that, if A=6 and B=8, then our equation (AB + B = BA) becomes 68 + 8 = 86 Since this equation is faulty, we can eliminate answer choice B Cheers, Brent

Choose how you want to prepare

Get Ready for GMAT Test Day Faster with Live Online Classes

Target Test Prep EA OnDemand

Target Test Prep GMAT OnDemand

A and B represent digits

GMAT/MBA Expert

GMAT/MBA Expert

GMAT/MBA Expert

GMAT/MBA Expert

Chicago Booth

GMAT Course Reviews

Admissions Consulting Reviews

FREE GMAT PREP RESOURCES

GMAT PREP DEAL TRACKER