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Integer Positive

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Integer Positive

by vinay1983 » Tue Aug 27, 2013 6:24 am
If n is a positive integer, what is the tens digit of n?

1. The hundredths digit of 10n is 6

2. The tens digit of n+1 is 7

For the second statement I thought that the number could have been 69 or numbers from 70 to 79.So i knew that we are not getting a single value,hence I chose A, which is correct?

I want to know how A is correct?
You can, for example never foretell what any one man will do, but you can say with precision what an average number will be up to!
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Source: — Data Sufficiency |

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by Brent@GMATPrepNow » Tue Aug 27, 2013 6:27 am
vinay1983 wrote:If n is a positive integer, what is the tens digit of n?

1. The hundredths digit of 10n is 6

2. The tens digit of n+1 is 7
Target question: What is the tens digit of n?

Statement 1: the hundreds digit of 10n is 6
Notice what happens when we multiply any positive integer by 10:
34 x 10 = 340
60 x 10 = 600
128 x 10 = 1280
54629 x 10 = 546290
The tens digit in the original number becomes the hundreds digit in the new number.

So, if we're told that the hundreds digit of 10n is 6, then we can conclude that the tens digit in n must be 6.
So, statement 1 is SUFFICIENT

Statement 2: the tens digit of n+1 is 7
There are several values of n that meet this condition. Here are two:
case a: n=69 in which case the tens digit of n is 6
case b: n=74 in which case the tens digit of n is 7
Since we cannot answer the target question with certainty, statement 2 is NOT SUFFICIENT

Answer: A

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Brent
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by [email protected] » Tue Aug 27, 2013 11:47 am
Hi vinay1983,

Brent's explanation is right-on, so I won't rehash it here. The issue in this question is basically a "decimal shift" issue, which is something you'll see on the GMAT as least once. In this DS question, we're asked to deal with 10 times a number, which is a clue that we're dealing with this type of math. You will also have to deal with this issue any time the answers having shifting decimals:

For example

210
21
2.1
.21
.021

This is essentially a test of your "accounting" ability - it's really easy to get this question correct and really easy to screw it up. To get the 700+ that everyone's after, you have to be perfect on these questions (in addition to all of the other questions that you need to get correct).

GMAT assassins aren't born, they're made,
Rich
Contact Rich at [email protected]
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