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xy = 10^n

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by Brent@GMATPrepNow » Sun Dec 28, 2008 12:23 pm
x and y are positive integers such that xy = 10^n. If x>y, the units digit of x-y CANNOT be
A) 1
B) 3
C) 5
D) 7
E) 9
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Re: xy = 10^n

by parallel_chase » Sun Dec 28, 2008 12:32 pm
Brent Hanneson wrote:x and y are positive integers such that xy = 10^n. If x>y, the units digit of x-y CANNOT be
A) 1
B) 3
C) 5
D) 7
E) 9
xy=10^n

x>y

x =5 , y= 2 , x-y = 3

x = 25, y = 4, x-y = 1

x = 10, y = 1, x-y = 9

x = 25, y = 8, x-y = 7

Hence 5.
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Re: xy = 10^n

by aroon7 » Sun Dec 28, 2008 12:47 pm
parallel_chase wrote:
Brent Hanneson wrote:x and y are positive integers such that xy = 10^n. If x>y, the units digit of x-y CANNOT be
A) 1
B) 3
C) 5
D) 7
E) 9
xy=10^n

x>y

x =5 , y= 2 , x-y = 3

x = 25, y = 4, x-y = 1

x = 10, y = 1, x-y = 9

x = 25, y = 8, x-y = 7

Hence 5.
x=200, y=5, x-y = 195. so not 5
Ans is 7
when x= 25, y=8, we get xy = 200. this is not nth power of 10...

what is the OA?

thanks!
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Re: xy = 10^n

by parallel_chase » Sun Dec 28, 2008 12:50 pm
aroon7 wrote:
parallel_chase wrote:
Brent Hanneson wrote:x and y are positive integers such that xy = 10^n. If x>y, the units digit of x-y CANNOT be
A) 1
B) 3
C) 5
D) 7
E) 9
xy=10^n

x>y

x =5 , y= 2 , x-y = 3

x = 25, y = 4, x-y = 1

x = 10, y = 1, x-y = 9

x = 25, y = 8, x-y = 7

Hence 5.
x=200, y=5, x-y = 195. so not 5
Ans is 7
when x= 25, y=8, we get xy = 200. this is not nth power of 10...

what is the OA?

thanks!

There was a typo


x = 125, y = 8, x-y = 7

xy = 1000 = 10^n


x = 200, y = 5, x-y = 5

I guess the correct option is missing or I am missing something ?
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by Brent@GMATPrepNow » Sun Dec 28, 2008 1:51 pm
Man, what a bad day for me.
I think this might be my last post, since I have now posted 2 questions with missing/incorrect information.

The question needs the proviso, "x and y" are not multiples of 10.
If we add that proviso, the correct answer is C (5).

Yikes - pretty sloppy stuff.
Sorry to all those who spent a great deal of time on this poorly-constructed question.
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by parallel_chase » Sun Dec 28, 2008 2:01 pm
Brent Hanneson wrote:Man, what a bad day for me.
I think this might be my last post, since I have now posted 2 questions with missing/incorrect information.

The question needs the proviso, "x and y" are not multiples of 10.
If we add that proviso, the correct answer is C (5).

Yikes - pretty sloppy stuff.
Sorry to all those who spent a great deal of time on this poorly-constructed question.
I hope these 2 questions dont stop you from drafting questions in future

2 bad out of 20 awesome questions- I am sure we can live with that :)

Its a pleasure having you on this forum.

Thanks.
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by Brent@GMATPrepNow » Sun Dec 28, 2008 2:05 pm
Thanks PC.
I'll continue to create questions, but perhaps I better quadruple check them in the future.
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by Ian Stewart » Sun Dec 28, 2008 2:16 pm
Brent Hanneson wrote:Man, what a bad day for me.
I think this might be my last post
I hope not- it's clear that many people here have appreciated your questions, and I think everyone here understands that designing questions can be a tricky thing to do, especially when a question doesn't follow a pre-existing model. One of the more interesting facts that I found reading the legal documents from the GMAC/ScoreTop lawsuit: to design one official GMAT question, GMAC spends, on average, $2400, which should give people some idea of how much work goes into this (and why it's so hard to emulate the real thing).
Brent Hanneson wrote: The question needs the proviso, "x and y" are not multiples of 10.
If we add that proviso, the correct answer is C (5).
As a suggestion, it might be more elegant to say that the GCD of x and y is 1, rather than to say that neither is a multiple of 10. It's a nice question, just as long as some kind of condition is added.
For online GMAT math tutoring, or to buy my higher-level Quant books and problem sets, contact me at ianstewartgmat at gmail.com

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by Brent@GMATPrepNow » Sun Dec 28, 2008 2:31 pm
I love your suggestion to note that the gcd=1. Much more eloquent than saying that neither value is a multiple of 10.

It reminds me of the popular GMAT proviso "xy is not equal zero," rather than saying "x is not equal to zero and y is not equal to zero"
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by Brent@GMATPrepNow » Sun Dec 28, 2008 2:38 pm
I just realized that I wrote "this might be my last post" when I meant to say "this might be my last post for the day"
"last post ever" is overly dramatic given the situation.
Yet another error :lol:
Looks like someone didn't get enough sleep last night.
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