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In-action problem set: 5. what is the units digit of y? 1)

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Intern
Joined: 07 Oct 2011
Posts: 30
In-action problem set: 5. what is the units digit of y? 1) [#permalink]

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10 Oct 2011, 22:07
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25% (medium)

Question Stats:

85% (01:22) correct 15% (00:19) wrong based on 20 sessions

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In-action problem set:

5. what is the units digit of y?

1) the units digit of y^2 = 1
2) the units digit of y does not equal 1..

here the OA is C

I think OA is wrong - there is no place where it says y is positive or negative
However, if you take y as a negative value, the ans should be E. What do you think?
[Reveal] Spoiler: OA
Manager
Joined: 20 Aug 2011
Posts: 146

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10 Oct 2011, 22:20
OA is correct.

karthiksms wrote:
I think OA is wrong - there is no place where it says y is positive or negative
However, if you take y as a negative value, the ans should be E. What do you think?

What difference does it make? Units digit is the last digit and that's what you're interested in.
let y be negative,

1.
last digit of y^2=1
eg. -9^=81
-11^2=121
that means units of digit of y can be 1 or 9.
1 is insufficient

2.
Insufficient as the digit can be 0,2,3,4,5,6,7,8, & 9

1+2
Between 1 & 9, 1 is not possible
Hence
digit is 9
So C
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Intern
Joined: 07 Oct 2011
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10 Oct 2011, 22:26
oh yeah..you're right..thanks
Manager
Joined: 20 Aug 2011
Posts: 146

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10 Oct 2011, 22:27
I know...
You're welcome
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Intern
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10 Oct 2011, 22:43
@blink005.. i see that you got a really high gmat score.. could you tell us what books you used for preparation for both quant and verbal.. thanks
Math Forum Moderator
Joined: 20 Dec 2010
Posts: 2010

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11 Oct 2011, 00:18
karthiksms wrote:
In-action problem set:

5. what is the units digit of y?

1) the units digit of y^2 = 1
2) the units digit of y does not equal 1..

here the OA is C

I think OA is wrong - there is no place where it says y is positive or negative
However, if you take y as a negative value, the ans should be E. What do you think?

Please hide the hints in a spoiler.

IMO, the answer should be E, not because y can be -ve or +ve but because y may not be an integer.

e.g.
y=457.56; y^2=209361.1536
y=9; y^2=81

Units digit can be 7 or 9. Not Sufficient.

Ans: "E"

What is the units digit of integer y?
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11 Oct 2011, 02:05
fluke wrote:
IMO, the answer should be E, not because y can be -ve or +ve but because y may not be an integer.

e.g.
y=457.56; y^2=209361.1536
y=9; y^2=81

Units digit can be 7 or 9. Not Sufficient.

Ans: "E"

What is the units digit of integer y?

Yeah, sorry about that. I overlooked that possibility. I had seen a similar question that stated y was an integer.

There seems to be a similar question-
http://www.manhattangmat.com/forums/uni ... 13490.html
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Math Forum Moderator
Joined: 20 Dec 2010
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11 Oct 2011, 04:11

Yeah, I have seen this. The answer should be "E" for this one too.
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11 Oct 2011, 09:11
karthiksms wrote:
In-action problem set:

5. what is the units digit of y?

1) the units digit of y^2 = 1
2) the units digit of y does not equal 1..

here the OA is C

I think OA is wrong - there is no place where it says y is positive or negative
However, if you take y as a negative value, the ans should be E. What do you think?

Unlike the question in the link, this question doesn't require different solutions for positive/negative numbers.
Statement 1 squares the expression.
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Director
Joined: 01 Feb 2011
Posts: 755

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11 Oct 2011, 16:17
units digit of y=?

1. Not Sufficient

units digit of y^2 = 1

=>units digit y can be 1 or 9 or 1.21 ..or 1.29...

2. Not Sufficient

units digit of y is not equal to 1.

units digit of y can be 0,2,3,.....9

Together, Not sufficient

y can still be 9 or 1.29 so on..

Manager
Joined: 20 Nov 2010
Posts: 218

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12 Oct 2011, 20:55
It should be E. As it is not mentioned that the number is an integer.
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http://gmatclub.com/forum/massive-collection-of-verbal-questions-sc-rc-and-cr-106195.html#p832142
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http://gmatclub.com/forum/gmat-prep-critical-reasoning-collection-106783.html
http://gmatclub.com/forum/how-to-get-6-0-awa-my-guide-64327.html
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