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# What is the value of s?

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Intern
Joined: 24 Aug 2017
Posts: 8
What is the value of s? [#permalink]

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11 Oct 2017, 21:01
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Difficulty:

35% (medium)

Question Stats:

71% (01:12) correct 29% (01:03) wrong based on 42 sessions

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What is the value of s?

1) $$\frac{1}{s} = \frac{1}{(s+25)} + \frac{1}{(s+1)}$$
2) -1/5 is not a reciprocal of s
[Reveal] Spoiler: OA
Math Expert
Joined: 02 Aug 2009
Posts: 5660
Re: What is the value of s? [#permalink]

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12 Oct 2017, 00:24
1
KUDOS
Expert's post
khalid228 wrote:
What is the value of s?

1) $$\frac{1}{s} = \frac{1}{(s+25)} + \frac{1}{(s+1)}$$
2) -1/5 is not a reciprocal of s

Let's see the statements
1) $$\frac{1}{s} = \frac{1}{(s+25)} + \frac{1}{(s+1)}$$
$$\frac{1}{s+25}+\frac{1}{s+1}-\frac{1}{s}=0........\frac{s^2+s+s^2+25s-(s^2+26s+25)}{s(s+1)(s+25)}=0.........$$
For the fraction to be 0, numerator has to be 0..
So $$s^2-25=0$$ or s=5 or s=-5
Two values possible..
Insufficient

2) -1/5 is not a reciprocal of s
s can be any value except-5
Insufficient

Combined..
Only value possible is s as 5
Sufficient

C
_________________

Absolute modulus :http://gmatclub.com/forum/absolute-modulus-a-better-understanding-210849.html#p1622372
Combination of similar and dissimilar things : http://gmatclub.com/forum/topic215915.html

BANGALORE/-

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Joined: 24 Aug 2017
Posts: 8
What is the value of s? [#permalink]

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12 Oct 2017, 00:39
chetan2u I understand the concept and ideal behind it now, but could you explain to me how you went from the first equation to the second?

$$\frac{1}{s+25}+\frac{1}{s+1}-\frac{1}{s}=0............. \frac{s^2+s+s^2+25s-(s^2+26s+25)}{s(s+1)(s+25)}=0$$
Math Expert
Joined: 02 Aug 2009
Posts: 5660
What is the value of s? [#permalink]

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12 Oct 2017, 19:18
khalid228 wrote:
chetan2u I understand the concept and ideal behind it now, but could you explain to me how you went from the first equation to the second?

$$\frac{1}{s+25}+\frac{1}{s+1}-\frac{1}{s}=0............. \frac{s^2+s+s^2+25s-(s^2+26s+25)}{s(s+1)(s+25)}=0$$

Hi..

We are actually getting all terms in equivalent form by getting the denominator same for all fractions
First take LCM of the denominator and then simplify the term by adding all fractions.

$$\frac{1}{s+25}+\frac{1}{s+1}-\frac{1}{s}=\frac{1*(s+1)*s+(s+25)*s-(s+1)(s+25)}{(s+1)(s+25)(s)}=0............. \frac{s^2+s+s^2+25s-(s^2+26s+25)}{s(s+1)(s+25)}=0$$[/quote]
_________________

Absolute modulus :http://gmatclub.com/forum/absolute-modulus-a-better-understanding-210849.html#p1622372
Combination of similar and dissimilar things : http://gmatclub.com/forum/topic215915.html

BANGALORE/-

Director
Joined: 17 Dec 2012
Posts: 627
Location: India
Re: What is the value of s? [#permalink]

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14 Oct 2017, 20:57
The second statement is a giveaway. It suggests we get two values from statement 1. Solving the first statement is also straightforward. We get 2 values for s. With statements 1 and 2, the question can be answered.

Hence C.
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Srinivasan Vaidyaraman
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http://www.sravnatestprep.com

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Re: What is the value of s?   [#permalink] 14 Oct 2017, 20:57
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