Problem of the Day


Let n, k,and y be positive integers such that y>n. If  k\cdot2^n=2^y+2^n, then k could equal all of the following except:

A. 33
B. 65
C. 123
D. 257
E. 513

Reveal Answer

Answer

C. 123

See the Solution

Solution

[latexpage]

$k\cdot2^n=2^y+2^n$

If $y>n$, then we can factor out a $2^n$ from the right side:

$k\cdot2^n=2^n(2^{y-n}+1)$

$k=2^{y-n}+1$

Notice that this says that $k$ is equal to one more than a power of 2. Every answer choices is one more than a power of two except 123.


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