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by runningintheair » Sat May 14, 2016 11:49 pm
If n is a positive integer, what is the tens digit of n?

(1) the hundreds digit of 10n is 6
(2) the tens digit of n+1 is 7

Answer is A.

why is statement 2 insufficient? isnt n=7-1 and hence 6 as the only answer?
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Source: — Data Sufficiency |

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by Brent@GMATPrepNow » Sun May 15, 2016 6:46 am
If n is a positive integer, what is the tens digit of n?

1. The hundreds digit of 10n is 6.
2. The 10s digit of n+1 is 7.
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 know that the tens digit in n must be 6.
So, statement 1 is SUFFICIENT

Statement 2: the tens digit of n+1 is 7
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 can't answer the target question with certainty, statement 2 is NOT SUFFICIENT and the answer is A

Cheers,
Brent
Brent Hanneson - Creator of GMATPrepNow.com
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by [email protected] » Sun May 15, 2016 9:29 am
Hi runningintheair,

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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