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# PS: Number property

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PS: Number property [#permalink]  22 Aug 2009, 11:09
00:00

Question Stats:

37% (01:39) correct 62% (00:42) wrong based on 8 sessions
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Q15:
Which of the following fractions has a decimal equivalent that is a terminating decimal?

A. 10/189
B. 15/196
C. 16/225
D. 25/144
E. 39/128
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Re: PS: Number property [#permalink]  22 Aug 2009, 11:35
Is it E? If I remember correctly, the fraction will terminate only if the denominator has only 2s or 5s in its prime factorization.
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Re: PS: Number property [#permalink]  23 Aug 2009, 02:50
Is there any rules to solve it quickly without actual calculation?
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Re: PS: Number property [#permalink]  23 Aug 2009, 08:27
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I do not think you have to calculate a lot here. The rule is that the fraction terminates if the denominator has ONLY 2 or/and 5 in its prime factorization.

A 10/189...no common factors can be canceled out, and 189 is divisible by 3. The denominator does not have ONLY 2's and 5's since it has 3 in it as well. You can stop here, concluding the fraction does not terminate...
B 15/196 - no common factors can be canceled out. 196=2*2*7*7...there is a prime number 7...so the fraction does not terminate..
C 16/225...no common factors can be caceled out. decomposing the denominator 225=25*9=5^2*3^2. Does not terminate since it has 3 in its denominator
D 25/144 no common factors can be canceled out. 144= 2*72, and 72 is divisible by 3...so the fraction has 3 in its denominator...does not terminate.
E you do not have to check. you know it should be E since it is your last option. But if you want to be sure decompose the denominator and you will get only 2's in it
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Re: PS: Number property [#permalink]  23 Aug 2009, 18:07
Yes, OA is E.

I think Lena's explanation is good.
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Re: PS: Number property [#permalink]  24 Aug 2009, 00:05
if the denominator has 3 or 7 in it and can't be reduct, it must be un-terminating.

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Re: PS: Number property [#permalink]  07 Sep 2010, 00:02
great explaination
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Re: PS: Number property [#permalink]  07 Sep 2010, 02:37
E. Great question.
Be suspicious about 3,7 (more suspicious if the numerator has a factor of 3 or 7). Do not blindly select the answer if the denominator alone has a 3 or 7.
If it is 8,4,2,6 then they are usually terminating.
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Re: PS: Number property [#permalink]  07 Sep 2010, 05:30
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TriColor wrote:
-----------------------------------------

Q15:
Which of the following fractions has a decimal equivalent that is a terminating decimal?

A. 10/189
B. 15/196
C. 16/225
D. 25/144
E. 39/128

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*5^3. 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 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.)

Questions testing this concept:
700-question-94641.html?hilit=terminating%20decimal
is-r-s2-is-a-terminating-decimal-91360.html?hilit=terminating%20decimal
pl-explain-89566.html?hilit=terminating%20decimal
which-of-the-following-fractions-88937.html?hilit=terminating%20decimal

BACK TO THE ORIGINAL QUESTION:

As 128=2^7, then \frac{39}{128} will be terminating decimal.

Hope it helps.
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Touch Question Qithout a Calculator [#permalink]  08 Sep 2010, 02:59
15). Which of the following fractions has a decimal equivalent that is a terminating decimal?
A. 10/189
B. 15/196
C. 16/225
D. 25/144
E. 39/128

I am interested in to know how to crack this without a calculator.

Cheers!
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Re: Touch Question Qithout a Calculator [#permalink]  08 Sep 2010, 03:30
Ans = E

the necessary and sufficient condition of terminating decimals is that the denominator's prime factors should only be 2 or 5 or both (form 2^x * 5^y)
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Re: Touch Question Qithout a Calculator   [#permalink] 08 Sep 2010, 03:30
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