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If x=0.abcabc........(a repeating infinite decimal), what is the value

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If x=0.abcabc........(a repeating infinite decimal), what is the value  [#permalink]

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New post 10 Oct 2018, 00:58
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[Math Revolution GMAT math practice question]

If x=0.abcabc........(a repeating infinite decimal), what is the value of a+b+c?

1) 1-x=0.123123123…….
2) 0.8<x<0.9

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Re: If x=0.abcabc........(a repeating infinite decimal), what is the value  [#permalink]

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New post 10 Oct 2018, 01:06
MathRevolution wrote:
[Math Revolution GMAT math practice question]

If x=0.abcabc........(a repeating infinite decimal), what is the value of a+b+c?

1) 1-x=0.123123123…….
2) 0.8<x<0.9


Answer: A.
This question can easily be solved using the Logical approach - no need to actually solve, just check if there is enough data to do so.
To find the value of a + b + c we want to find the value of x (which will give us the value of all 3 variables).
(1) is an equation with one variable - solving it will give us x, which will give us a, b, and c. sufficient! eliminate B, C & E.
(2) gives us a range for x, which tells us that a=8 - but we have no idea what b and c are! not sufficient! D is eliminated

Answer A
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Re: If x=0.abcabc........(a repeating infinite decimal), what is the value  [#permalink]

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New post 10 Oct 2018, 01:08
From statement 1:

1-x = 0.123123123.

x = 1-0.123123123 = 0.876876.

So a+b+c = 21.
A is sufficient.

From statement 2:

This is clearly insufficient.

A is the answer.
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Re: If x=0.abcabc........(a repeating infinite decimal), what is the value  [#permalink]

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New post 12 Oct 2018, 08:32
=>
Forget conventional ways of solving math questions. For DS problems, the VA (Variable Approach) method is the quickest and easiest way to find the answer without actually solving the problem. Remember that equal numbers of variables and independent equations ensure a solution.

Since we have 4 variables (x, a, b and c) and 1 equation (x = 0.abcabc….), E is most likely to be the answer. However, condition 1) includes 3 hidden equations (matching the values of the decimal places) and allows us to determine the values of all four variables as follows:
x = 1 – 0.123123123… = 0.876876876… = 0.abcabcabcabc…
Thus a = 8, b = 7, c = 6 and a + b + c = 8 + 7 + 6 = 21.
Condition 1) is sufficient.

Condition 2)
If x = 0.811811811…, then a = 8, b = 1, c = 1 and a + b + c = 10.
If x = 0.812812812…, then a = 8, b = 1, c = 2 and a + b + c = 11.
Since we don’t have a unique solution, condition 2) is not sufficient.


Therefore, A is the answer.
Answer: A
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Re: If x=0.abcabc........(a repeating infinite decimal), what is the value  [#permalink]

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New post 12 Oct 2018, 13:49
MathRevolution wrote:
[Math Revolution GMAT math practice question]

If x=0.abcabc........(a repeating infinite decimal), what is the value of a+b+c?

1) 1-x=0.123123123…….
2) 0.8<x<0.9

\(x = 0.\overline {abc} \,\,\,\,\left( {{\rm{common}}\,\,{\rm{russian}}\,\,{\rm{notation}}} \right)\)

\(? = a + b + c\)

\(\left( 1 \right)\,\,\,1 - x = 0.\overline {123} = {{123} \over {999}}\,\,\,\,\,\,\,\, \Rightarrow \,\,\,\,\,\,\,x = {{876} \over {999}} = 0.\overline {876} \,\,\,\,\,\,\,\,\,\, \Rightarrow \,\,\,\,\,\,\,\,? = 8 + 7 + 6\)

\(\left( 2 \right)\,\,0.8 < x < 0.9\,\,\,\,\,\,\left\{ \matrix{
\,{\rm{Take}}\,\,x = 0.\overline {801} \,\,\,\,\, \Rightarrow \,\,\,? = 8 + 0 + 1\,\, \hfill \cr
\,{\rm{Take}}\,\,x = 0.\overline {802} \,\,\,\,\, \Rightarrow \,\,\,? = 8 + 0 + 2 \hfill \cr} \right.\)


This solution follows the notations and rationale taught in the GMATH method.

Regards,
Fabio.
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Re: If x=0.abcabc........(a repeating infinite decimal), what is the value  [#permalink]

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New post 12 Oct 2018, 14:19
MathRevolution wrote:
[Math Revolution GMAT math practice question]

If x=0.abcabc........(a repeating infinite decimal), what is the value of a+b+c?

1) 1-x=0.123123123…….
2) 0.8<x<0.9



We need a unique value.

Statement 1)

1 - x = 0.123123123

Gives us a unique value for x. Sufficient.

Statement 2)

0.8 < x < 0.9

x could be anything so we cannot tell the value.

Insufficient.

Answer choice A

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Re: If x=0.abcabc........(a repeating infinite decimal), what is the value   [#permalink] 12 Oct 2018, 14:19
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