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BrentGMATPrepNow
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BrentGMATPrepNow
\(K = 10^{26} + 2^{26}\), and \(K\) is a multiple of \(2^n\) but NOT a multiple of \(2^{n+1}\). If n is a positive integer, what is the value of \(n+1\)?

A) 25
B) 26
C) 27
D) 28
E) 29

IMPORTANT: later in my solution, we need to recognize that 5^b will end in 25 for all integer values of b greater than 0.
For example, 5^2 = 25
5^3 = 125
5^4 = 625
5^5 = 3125
5^6 = XXX25 etc....

So......
K = 10^26 + 2^26
= 2^26(5^26 + 1)
= 2^26(XXXX25 + 1) [aside: XXXX25 denotes some number ending in 25]
= 2^26(XXXX26)
= 2^26[2(XXXX3)] [Since XXX26 is EVEN, we can factor out a 2]
= 2^27[XXXX3]

Since XXXX3 is an ODD number, we can't factor out any more 2's.
This means K is a multiple of 2^27, but K is NOT a multiple 2^28.
In other words, n = 27

What is the value of n + 1?
Since n = 27, we can conclude that n + 1 = 27 + 1 = 28

Answer: D

Cheers,
Brent
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2^26(5^26+1)

Now 5^26+1 is divisible by 2

This is a multiple of 2^27

N+1 = 28

D
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­Could anyone please explain how we can say with certainity that (\(5^{26}\)+1) will not have a power of 2 greater than 1. Thanks!­
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vraju87
­Could anyone please explain how we can say with certainity that (\(5^{26}\)+1) will not have a power of 2 greater than 1. Thanks!­
Good question.
­
First of all:

Divisibility rule for \(2^n\):

An integer is divisible by 2 if its last digit is divisible by 2.

An integer is divisible by 4 if its last two digits is divisible by 4.

An integer is divisible by 8 if its last three digits is divisible by 8.

...

Generally, an integer is divisible by \(2^n\) if its last \(n\) digits is divisible by \(2^n\).
Next, 5 raised to any positive integer power greater than 1 ends with 25. Thus, 5^26 + 1 = ...25 + 1 = ...26. Since 26 is not divisible by 4, then 5^26 + 1 is not divisible by any power of 2 greater than 1.

Hope it's clear.­
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vraju87
­Could anyone please explain how we can say with certainity that (\(5^{26}\)+1) will not have a power of 2 greater than 1. Thanks!­
Good question.
­
First of all:


Divisibility rule for \(2^n\):

An integer is divisible by 2 if its last digit is divisible by 2.

An integer is divisible by 4 if its last two digits is divisible by 4.

An integer is divisible by 8 if its last three digits is divisible by 8.

...

Generally, an integer is divisible by \(2^n\) if its last \(n\) digits is divisible by \(2^n\).
Next, 5 raised to any positive integer power greater than 1 ends with 25. Thus, 5^26 + 1 = ...25 + 1 = ...26. Since 26 is not divisible by 4, then 5^26 + 1 is not divisible by any power of 2 greater than 1.

Hope it's clear.­
Thank you for the clear explanation!
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