unique odd digits

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umaa: Legendary Member; Posts: 727; Joined: Sun Jun 08, 2008 9:32 pm; Thanked: 8 times; Followed by:1 members

unique odd digits

by umaa » Mon Jul 21, 2008 6:22 am

Say A, B and C represent unique odd digits. Then take a look at the following multiplication problem: ABA * B = CBB. Which of these can represent the number ABA?

A. 171
B. 353
C. 313
D. 151
E. 121

Please let me know the answer

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Source: Beat The GMAT — Problem Solving | Mon Jul 21, 2008 6:22 am

reachac: Legendary Member; Posts: 572; Joined: Fri Feb 22, 2008 9:25 am; Thanked: 21 times

by reachac » Mon Jul 21, 2008 6:31 am

IMO D

Just plugged in the options.

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VP_Tatiana: GMAT Instructor; Posts: 189; Joined: Thu May 01, 2008 10:55 pm; Location: Seattle, WA; Thanked: 25 times; Followed by:1 members; GMAT Score:750+

by VP_Tatiana » Mon Jul 21, 2008 6:35 am

We want to figure out which number, when multiplied by the single digit in its tens position, will result in:

1) A 3 digit number
2) A number with the same units and tens digit

I notice that B and D have tens digit 5. I think they are good possibilities to fit the bill, since 5*odd number always ends in 5 (and we know all of the digits are odd.) This makes it highly likely that the last two digits will be the same.

First I try B, 353. I want to work out 353*5. I remember that 333.333*3 = 1000. Since both parts of the product 353*5 are bigger than that, we know this results in something over 1000. This does not fit the bill since it is not a 3 digit number.

Now I try D, 151. I line up the multiplication problem:

151
X 5

I see I get a 5 in the units digit, then a 5 in the tens digit... I don't need to worry about carrying the 2 or going any further because I know this is a 3 digit number whose last two digits are the same.

So, the answer is D.

Tatiana Becker | GMAT Instructor | Veritas Prep

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umaa: Legendary Member; Posts: 727; Joined: Sun Jun 08, 2008 9:32 pm; Thanked: 8 times; Followed by:1 members

by umaa » Mon Jul 21, 2008 6:35 am

can you please explain me. I'm getting 171.

19*3*3

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parallel_chase: Legendary Member; Posts: 1153; Joined: Wed Jun 20, 2007 6:21 am; Thanked: 146 times; Followed by:2 members

Re: unique odd digits

by parallel_chase » Mon Jul 21, 2008 6:37 am

umaa wrote:Say A, B and C represent unique odd digits. Then take a look at the following multiplication problem: ABA * B = CBB. Which of these can represent the number ABA?

A. 171
B. 353
C. 313
D. 151
E. 121

Please let me know the answer

The answer is 151, you simply have to look at the answer options.

Lets start with 171*7=something 9 7

353*5 = something 6 5

151 * 5 = 755
ABA * B = CBB

The long way is ABA*B= CBB

A*B = B has to be in the units place, therefore A has to be 1

1B1 * B = CBB

B*B = B is in the units place, therefore only integer possible is 5

151*5 = C55

I hope this clears everything.