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Bunuel
S is the set of all fractions defined by the formula n/(n + 1), where n is a positive integer less than 20. What is the product of all the fractions that are in S?

A. 1/20
B. 1/21
C. 1/2
D. 19/40
E. 19/20

t1=1/2
t2=2/3
t3=3/4

t19= 19/20
t1*t2*t3...t19= 1/20 A
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Bunuel
S is the set of all fractions defined by the formula n/(n + 1), where n is a positive integer less than 20. What is the product of all the fractions that are in S?

A. 1/20
B. 1/21
C. 1/2
D. 19/40
E. 19/20

Given n is an integer and 0<n<20

So the set will contain the terms when n=1 till n=19
Let t(n) = n/n+1

t(1) = 1/2
t(2) = 2/3
t(3) = 3/4
t(4) = 4/5
.
.
.
.
t(19) = 19/20

Product of all the terms \(=\frac{1}{2} * \frac{2}{3} * \frac{3}{4} * \frac{4}{5} .............\frac{19}{20} = \frac{1}{20}\)

Correct ans : A
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Bunuel
S is the set of all fractions defined by the formula n/(n + 1), where n is a positive integer less than 20. What is the product of all the fractions that are in S?

A. 1/20
B. 1/21
C. 1/2
D. 19/40
E. 19/20

We need to evaluate the following product:

1/2 x 2/3 x 3/4 x … x 18/19 x 19/20

However, let’s look at the product of the first 3 values:

1/2 x 2/3 x 3/4 = 1/4

We see that only the first numerator and last denominator remain; thus, the product of all the fractions that are in S is 1/20.

Answer: A
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Different approach than what I see posted so far:

1/2 * 2/3 * 3/4 *4/5 ... etc 19/20.

numerator = 19!
denominator = 20!

simplifies to 1/20
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