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Challenge Problem - April 18 2011

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ambikasrinivas
 Post subject: Challenge Problem - April 18 2011
 Post Posted: Sat May 21, 2011 8:39 pm 
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Course Students


Posts: 13
Hi - I understand the answer for this question, but I am having issues with the stem of the question:

1101 = (1 × 2^3) + (1 × 2^2) + (0 × 2^1) + (1 × 2^0) = 25

When I add this, I get 13: 8 + 4 + 0 + 1

What am I missing?

Thanks!

Question
Every digit of a number written in binary is either 0 or 1. To translate a number from binary, multiply the nth digit (reading from right to left) by 2^n – 1. For instance, 1101 = (1 × 2^3) + (1 × 2^2) + (0 × 2^1) + (1 × 2^0) = 25. What is the largest prime number (written in binary) that is a factor of both 100010000 and 1000100000 ?
(A) 10
(B) 11
(C) 101
(D) 1011
(E) 10001
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shoumik
 Post subject: Re: Challenge Problem - April 18 2011
 Post Posted: Tue May 24, 2011 12:32 pm 
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Forum Guests


Posts: 12
I think there is a mistake. I am pretty sure the binary of 1101 = 13. Hmm..
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jnelson0612
 Post subject: Re: Challenge Problem - April 18 2011
 Post Posted: Mon May 30, 2011 1:29 pm 
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ManhattanGMAT Staff


Posts: 1857
I get 13 also. I agree with you both that 25 is an error.

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Jamie Nelson
ManhattanGMAT Instructor
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