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Units digit problem - is MGMAT answer key wrong?

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leong_edwin
 Post subject: Units digit problem - is MGMAT answer key wrong?
 Post Posted: Wed Aug 03, 2011 7:12 pm 
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In the advanced MGMAT quant book, I came across the following problem:

What is the units digit of y?

1) The units digit of y^2 is 6
2) The units digit of (y+1)^2 is 5

Answer key says B is right answer.


I agree that A and D are incorrect, due to fact that y can be either 4 or 6 for statement 1.

But for statement 2, we know that (y+1) needs sum to a number that has a ones digit of 5. Doesn't this mean that y can be either 4 or -6, which will both lead to either 5 or -5, which when squared will be 25. Doesn't this mean that the ones digit for y could be 4 or 6, so the answer should be E?

Any one have any insight on this?
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jnelson0612
 Post subject: Re: Units digit problem - is MGMAT answer key wrong?
 Post Posted: Sat Aug 06, 2011 6:35 pm 
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ManhattanGMAT Staff


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leong_edwin wrote:
In the advanced MGMAT quant book, I came across the following problem:

What is the units digit of y?

1) The units digit of y^2 is 6
2) The units digit of (y+1)^2 is 5

Answer key says B is right answer.


I agree that A and D are incorrect, due to fact that y can be either 4 or 6 for statement 1.

But for statement 2, we know that (y+1) needs sum to a number that has a ones digit of 5. Doesn't this mean that y can be either 4 or -6, which will both lead to either 5 or -5, which when squared will be 25. Doesn't this mean that the ones digit for y could be 4 or 6, so the answer should be E?

Any one have any insight on this?


Wow, I can't disagree with anything you've said and think you are right. The problem should have stipulated that y is positive.

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Jamie Nelson
ManhattanGMAT Instructor
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parleybaldwin
 Post subject: Re: Units digit problem - is MGMAT answer key wrong?
 Post Posted: Thu Aug 11, 2011 2:08 am 
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Perhaps when referring to "units digit," we are to assume that it is a positive number. Or maybe MGMAT just made that assumption. It could just be one of the quirks of the GMAT that we have to learn. Stupid test!...lol
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jnelson0612
 Post subject: Re: Units digit problem - is MGMAT answer key wrong?
 Post Posted: Fri Aug 19, 2011 6:53 pm 
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ManhattanGMAT Staff


Posts: 1857
parleybaldwin wrote:
Perhaps when referring to "units digit," we are to assume that it is a positive number. Or maybe MGMAT just made that assumption. It could just be one of the quirks of the GMAT that we have to learn. Stupid test!...lol


Well, a digit itself is always positive, thus the units digit of -15 is 5. I still think we have trouble here--hard to say!

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Jamie Nelson
ManhattanGMAT Instructor
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