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05 Jun 2015, 06:09
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D
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Difficulty:
55%
(hard)
Question Stats:
65% (01:50) correct
35%
(02:03)
wrong
based on 805
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Let x and y be positive integers, and r and s be single-digit positive integers. If x/y = r.sss, where the bar below the s indicates that the decimal repeats infinitely, which of the following CANNOT be true?
(A) y = 1.2 × 10^a, where a is a positive integer.
(B) y = 1.5 × 10^b, where b is a positive integer.
(C) y = 1.8 × 10^c, where c is a positive integer.
(D) y = 2.5 × 10^d, where d is a positive integer.
(E) y = 2.7 × 10^e, where e is a positive integer.
(A) y = 1.2 × 10^a, where a is a positive integer.
(B) y = 1.5 × 10^b, where b is a positive integer.
(C) y = 1.8 × 10^c, where c is a positive integer.
(D) y = 2.5 × 10^d, where d is a positive integer.
(E) y = 2.7 × 10^e, where e is a positive integer.
08 Jun 2015, 05:41
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Bunuel
MANHATTAN GMAT OFFICIAL SOLUTION:
Fractions that have only factors of 2 and 5 in the denominator equate to terminating decimals. Since x/y = r.sss, a non-terminating decimal, y must have some other prime factors besides just 2 and/or 5.
(A) y = 12, 120, 1,200, etc. Prime factors of 12: (2)(2)(3)
(B) y = 15, 150, 1,500, etc. Prime factors of 15: (3)(5)
(C) y = 18, 180, 1,800, etc. Prime factors of 18: (2)(3)(3)
(D) y = 25, 250, 2,500, etc. Prime factors of 25: (5)(5). CANNOT be true—only has 5's and 2's.
(E) y = 27, 270, 2,700, etc. Prime factors of 27: (3)(3)(3)
The correct answer is D.
06 Jun 2015, 01:00
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For such questions... generally one rule works most of the time
if there is only 2 or 5 or combination of 2 and 5 in denominator, it will always give u a fixed output rather than a never ending one.
You can try few examples
33/5 = 6.6
3333333333/5 = 666666.6 something
1111/2 = 555.5
in all options except D, there is a factor of 3 involved... where as in D only 5 is there (5*5 = 25)
in GMAT you dont need to solve every expression... there might be possibility of other options as correct but first skim through options and then use logic.
I have solved numerous questions of gmat but never felt the need to solve anything for such questions..
yes, if still you want to use logic, then a perfect algebric solution is provided by GMATINSIGHT
if there is only 2 or 5 or combination of 2 and 5 in denominator, it will always give u a fixed output rather than a never ending one.
You can try few examples
33/5 = 6.6
3333333333/5 = 666666.6 something
1111/2 = 555.5
in all options except D, there is a factor of 3 involved... where as in D only 5 is there (5*5 = 25)
in GMAT you dont need to solve every expression... there might be possibility of other options as correct but first skim through options and then use logic.
I have solved numerous questions of gmat but never felt the need to solve anything for such questions..
yes, if still you want to use logic, then a perfect algebric solution is provided by GMATINSIGHT
