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GMAT Official Guide 2019 If x is a positive integer

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by GMATGuruNY » Sat Jun 30, 2018 1:19 am
BTGmoderatorDC wrote:If x is a positive integer and $$4^x$$-3=y, which of the following CANNOT be a value of y ?


A. 1

B. 7

C. 13

D. 61

E. 253
Subtract 3 from increasing powers of 4:
4¹ - 3 = 1
4² - 3 = 13
The results above indicate that the following case is not possible:
$$4^x - 3 = 7$$

The correct answer is B.
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by Jeff@TargetTestPrep » Mon Jul 30, 2018 10:30 am
BTGmoderatorDC wrote:If x is a positive integer and $$4^x$$-3=y, which of the following CANNOT be a value of y ?


A. 1

B. 7

C. 13

D. 61

E. 253
Since 4^1 - 3 = 1 and 4^2 - 3 = 13, we see that y can't be 7.

Answer: B

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