sjuniv32
Which of the following fractions has a decimal equivalent that is a terminating decimal?
(A) \(\frac{71}{152}\)
(B) \(\frac{17}{81}\)
(C) \(\frac{117}{152}\)
(D) \(\frac{71}{512}\)
(E) \(\frac{17}{215}\)
THEORY:Reduced fraction \(\frac{a}{b}\) (meaning that fraction is already reduced to its lowest term) can be expressed as terminating decimal
if and only \(b\) (denominator) is of the form \(2^n5^m\), where \(m\) and \(n\) are non-negative integers. For example: \(\frac{7}{250}\) is a terminating decimal \(0.028\), as \(250\) (denominator) equals to \(2*5^2\). Fraction \(\frac{3}{30}\) is also a terminating decimal, as \(\frac{3}{30}=\frac{1}{10}\) and denominator \(10=2*5\).
Note that if denominator already has only 2-s and/or 5-s then it doesn't matter whether the fraction is reduced or not.
For example \(\frac{x}{2^n5^m}\), (where x, n and m are integers) will always be the terminating decimal.
We need reducing in case when we have the prime in denominator other then 2 or 5 to see whether it could be reduced. For example fraction \(\frac{6}{15}\) has 3 as prime in denominator and we need to know if it can be reduced.
BACK TO THE QUESTION:Only option D (when reduced to its lowest form) has the denominator of the form \(2^n5^m\): \(\frac{71}{512}=\frac{71}{2^9}\).
Answer: D.
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