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S is the set of all fractions [#permalink]
Expert's post 00:00

Question Stats: 38% (00:37) correct 61% (00:56) wrong based on 85 sessions
S is the set of all fractions of the form $$\frac{n}{n+1}$$, where n is a positive integer less than 20.

 Quantity A Quantity B The product of all the fractions that are in S $$\frac{1}{20}$$

A)The quantity in Column A is greater.
B)The quantity in Column B is greater.
C)The two quantities are equal.
D)The relationship cannot be determined from the information given.
[Reveal] Spoiler: OA

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Re: S is the set of all fractions [#permalink]
2
KUDOS
From the given expression,
1st fraction = $$1/2$$
2nd fraction = $$2/3$$
3rd fraction = $$3/4$$

Here we can observe a $$pattern$$
the numerator begins with 1 and ultimately will end in 19
the denominator begins with 2 and ultimately will end in 20

when we multiply all the fractions our end product will be $$\frac{1*2*3*4*5.........19}{2*3*4*5.......20}$$At this point we can cancel off all the numerators except 1 with all the denominators except 20 finally reducing the term to $$1/20$$
option c
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Re: S is the set of all fractions [#permalink]
1
KUDOS
Expert's post
Carcass wrote:
S is the set of all fractions of the form $$\frac{n}{n+1}$$, where n is a positive integer less than 20.

 Quantity A Quantity B The product of all the fractions that are in S $$\frac{1}{20}$$

Let's get a better idea of what Set S looks like.
We're told that n can have any integer value from 1 to 19 inclusive.

When n = 1, the corresponding fraction is $$\frac{1}{1+1}=\frac{1}{2}$$

When n = 2, the corresponding fraction is $$\frac{2}{2+1}=\frac{2}{3}$$

When n = 3, the corresponding fraction is $$\frac{3}{3+1}=\frac{3}{4}$$
.
.
.
.
.
When n = 18, the corresponding fraction is $$\frac{18}{18+1}=\frac{18}{19}$$

When n = 19, the corresponding fraction is $$\frac{19}{19+1}=\frac{19}{20}$$

So, Quantity B $$= (\frac{1}{2})(\frac{2}{3})(\frac{3}{4})(\frac{4}{5})......(\frac{17}{18})(\frac{18}{19})(\frac{19}{20})$$

$$=\frac{(1)(2)(3)(4)(5)....(17)(18)(19)}{(2)(3)(4)(5)....(17)(18)(19)(20)}$$

$$=\frac{1}{20}$$

We get:
QUANTITY A: $$\frac{1}{20}$$

QUANTITY B: $$\frac{1}{20}$$

Cheers,
Brent
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Brent Hanneson – Creator of greenlighttestprep.com  Retired Moderator
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Re: S is the set of all fractions [#permalink]
1
KUDOS
A -> (1/2) (2/3) (3/4) .....(19/20)
Cancelling the denominator of the term with the numerator of next term
= 1/20

A = B Retired Moderator
Joined: 16 Sep 2019
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Re: S is the set of all fractions [#permalink]
2
KUDOS
When n = 1 the fraction is $$\frac{1}{2}$$.
Now, A says product of all the fractions that are in S.
It means $$\frac{1}{2}*\frac{2}{3}*.....\frac{19}{20}$$ Since N is less than 20.
The denominator and numerator will cancel out and we will be left with $$\frac{1}{20}$$

Hence, C
Carcass wrote:
S is the set of all fractions of the form $$\frac{n}{n+1}$$, where n is a positive integer less than 20.

 Quantity A Quantity B The product of all the fractions that are in S $$\frac{1}{20}$$

A)The quantity in Column A is greater.
B)The quantity in Column B is greater.
C)The two quantities are equal.
D)The relationship cannot be determined from the information given.

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If you like my solution, do give kudos! Re: S is the set of all fractions   [#permalink] 26 Dec 2019, 20:41
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