ABC+BCB= CDD

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ABC+BCB= CDD

by pareekbharat86 » Tue Nov 19, 2013 12:46 am
ABC+BCB= CDD

In the addition shown above, A, B, C, and D represent the nonzero digits of three 3-digit numbers. What is the largest possible value of the product of A and B ?

a.8
b.10
c.12
d.14
e.18

OA is B.

Source- MGMAT Practice Test
Last edited by pareekbharat86 on Tue Nov 19, 2013 12:59 am, edited 1 time in total.
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by [email protected] » Tue Nov 19, 2013 12:58 am
Hi Bharat,

Have you transcribed this question correctly?

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by pareekbharat86 » Tue Nov 19, 2013 1:00 am
[email protected] wrote:Hi Bharat,

Have you transcribed this question correctly?

GMAT assassins aren't born, they're made,
Rich
Done the needful.
Thanks,
Bharat.

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by GMATGuruNY » Tue Nov 19, 2013 4:39 am
pareekbharat86 wrote:ABC+BCB= CDD

In the addition shown above, A, B, C, and D represent the nonzero digits of three 3-digit numbers. What is the largest possible value of the product of A and B ?

a.8
b.10
c.12
d.14
e.18
ABC
BCB
CDD

To MAXIMIZE A and B, we must MAXIMIZE the value of C.

Case 1: C=9
AB9
B9B
9DD
Here, B=0, which violates the constraint that B is nonzero.

Case 2: C=8
AB8
B8B
8DD
Here, B=1, implying that A=7.
In this case, A*B = 7*1, which is not among the answer choices.

Case 3: C=7
AB7
B7B
7DD
Here, it's possible that B=2, implying that A=5.
In this case, A*B = 5*2 = 10.

Case 4: C=6
AB6
B6B
6DD
Here:
If B=3, then A=3, in which case A*B = 3*3 = 9.
If B=2, then A=4, in which case A*B = 4*2 = 8.
If B=1, then A=5, in which case A*B = 5*1 = 6.

At this point, we can see that the maximum possible product is yielded by Case 3:
A*B = 5*2 = 10.

The correct answer is B.
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by Mathsbuddy » Tue Nov 19, 2013 7:59 am
ABC+BCB= CDD

A*B max = ?

The maximum value of 3 digits is 999

Therefore digit by digit, A + B <= 9 and B + C <= 9

Hence in the addition, no numbers are carried over from column to column.

This means we can ignore the third column as it is the same as the second:

ABC+BCB= CDD
1st column: A+B = C
2nd column: B+C = D
3rd column: C+B = D (redundant)

[Hence the question can be reduced to either AB + BC = CD or BA + CB = DC]

Equation 1: A+B = C
Equation 2: B+C = D

Substitute Eq1 into Eq2 gives:

A + 2B <= 9

A*B + 2B^2 <= 9B

Let A*B product = P

P = 9B - 2B^2

dP/dB = 9 - 4B = 0 when B = 9/4 = 2.25

d2P/dB2 = -4 < 0 so B = 2.25 is maximum

The closest integer is B = 2

Test B = 2:

A + 2*2 <= 9

A <= 5

A*B maximum = 5 * 2 = 10

I know that the calculus here is overkill. It would be quicker just to test the given values.
However, it's always a pleasure when you can share another method and it works!

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by Resp007 » Tue Nov 19, 2013 7:29 pm
How about a 3rd approach?
:)

Here it is:

Since

A B C
B C B
------------
C D D

Therefore, A + B = C and B + C = D; where each is a single digit no.

Add both the equations:

We get: A + B + B + C = C + D ----> A + 2B = D

Now since D is a single digit, therefore, A + 2B <= 9 and we need to maximize A*B.

So plug in the values for A and B in A + 2B <= 9 equation and check A*B maximizing.

1. When B = 1; we get A = 7, and product = 7

2. When B = 2; we get A = 5, and product = 10

3. When B = 3; we get A = 3, and product = 9

4. When B = 4; we get A = 1, and product = 4

5. When B = 5 or more then A + 2B <= 9 doesn't hold true, so our ans is in case #2

Ans: B

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by theCodeToGMAT » Tue Nov 19, 2013 10:29 pm
Another Approach ;)

A+B = C -(1)
B+C = D -(2)
C+B = D

Subtract (1) and (2)
A + B - B - C = C - D
A + D = 2C
A cannot be bigger than C
So, AP series is "A, C, D"

Also, A + B = C ==> B = C - D

For pattern of A,C,D as 7,8,9 ==> AB = (7)(1) = 7
For pattern of A,C,D as 5,7,9 ==> AB = (5)(2) = 10
For pattern of A,C,D as 3,6,9 ==> AB = (3)(3) = 9
So, [spoiler]{B}[/spoiler]
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