If N = 5/6 + 6/7 + 7/8 + 8/9 + 9/10 + 10/11, then which of the followi

Question

If  N = \frac{5}{6} + \frac{6}{7} + \frac{7}{8} + \frac{8}{9} + \frac{9}{10} + \frac{10}{11} , then which of the following must be true of N?

A) 5/6 < N < 10/11

B) 21/10 < N < 9/2

C) 5 < N < 5 1/11

D) 5 < N < 5 5/11

E) 30 < N < 60

amanvermagmat · Accepted Answer

Here's another way to do this:

N = 5/6 + 6/7 + 7/8 + 8/9 + 9/10 + 10/11

We should also know that in proper fractions (where numerator < denominator), adding same value to both numerator and denominator  increases  the value of fraction (eg, if we add 1 to both numerator and denominator of 5/6, we will get (5+1)/(6+1) = 6/7, which will become greater than 5/6). 
Hence 5/6 < 6/7 < 7/8 < 8/9 < 9/10 < 10/11

Now if we replace all 6 terms with the smallest term 5/6, of course it will decrease the value of given number N. But still lets see what happens
5/6 + 5/6 + 5/6 + 5/6 + 5/6 + 5/6 = 5/6 * 6 = 5

But N must be more than this (the way we replaced), so N > 5

Now lets replace all 6 terms with the highest term 10/11, this will of course increase the value of N but lets see what happens
10/11 + 10/11 + 10/11 + 10/11 + 10/11 + 10/11 = 10/11 * 6 = 60/11 = 5 5/11

But N must be less than this (the way we replaced), so N < 5 5/11

Combining we get 
5 < N < 5 5/11

Hence  D answer

0ld · Answer

N = \frac{5}{6} + \frac{6}{7} + \frac{7}{8} + \frac{8}{9} + \frac{9}{10} + \frac{10}{11}

N = 1 - \frac{1}{6} + 1 - \frac{1}{7} + 1 - \frac{1}{8} + 1 - \frac{1}{9} + 1 - \frac{1}{10} + 1 - \frac{1}{11}

N = 6 - (\frac{1}{6} + \frac{1}{7} + \frac{1}{8} + \frac{1}{9} + \frac{1}{10} + \frac{1}{11})

\frac{1}{6} + \frac{1}{7} + \frac{1}{8} + \frac{1}{9} + \frac{1}{10} + \frac{1}{11}

Max value of above fraction will be   \frac{6}{6} =  1

Minimum value =   \frac{6}{11}

6-1 <= N <= 6- \frac{6}{11}

6-1 <= N <=  \frac{60}{11}

6-1 <= N <= 5 \frac{5}{11}

Ans. D

Joc456 · Answer

Each fraction is close to 1 so we know that the sum will be less than 6. Lets just say they're each 1 so N=6. But now we have to subtract the difference
1/6+1/7+1/8+1/9+1/10+1/11. We can estimate these each as 1/10-->5/10 which is 1/2. D is the closest representation of this.

damybox · Answer

I think it isn't a matter of which is the "closest" approximation of N because the question asks which one MUST be true. Therefore the right answer may be even a statement which is not a close approximation of N (it just has to be true), whereas a good approximation of N may be the wrong answer. as a consequence i belive that the quickest and most certain way to answer this question in by exclusion of the impossible answers. A) FALSE_____ N<10/11 is impossible because N=10/11+ a lot of other numbers, thus N must be >10/11 B) FALSE_____N<9/2=4.5 is impossible because each of the 6 numbers which constitute N are very close to 1( So it is between 5 and 6 and >4.5) C)FALSE____ 51/11 <5 , whereas N is clearly >5 D) NO SENSE! because 55/11=5 therefore 5

mandeey · Answer

N= \frac{5}{6} + \frac{6}{7} + \frac{7}{8} + \frac{8}{9} + \frac{9}{10} + \frac{10}{11}

Lowest term =  \frac{5}{6} 
What if all the terms are equal to lowest term, then sum =  \frac{5}{6}  * 6 = 5

Highest term =  \frac{10}{11} 
What if all the terms are equal to highest term, then sum =  \frac{10}{11} *6 =  \frac{60}{11} 
 
Sum of terms will lie between sum of terms if all terms are equal to lowest terms and sum terms if all terms are equal to highest terms

5 < N < 60/11

Ans. D

hellosanthosh2k2 · Answer

5/6 < 1 and 6/7 will be more closer to 1 than 5/6 is

so, min value of the addition (all elements are 5/6) will be : 6 * (5/6) = 5
so, max value of the addition (all elements are 10/11) will be : 6 * (10/11) = 60/11 = 5 5/11

so the required addition will be min < N < max => 5 < N < 5 5/11 => (D)

destinyawaits · Answer

lolwut the answer is that which you labelled as making no sense...

GMAT2645 · Answer

How can you estimate each of the fractions as 1/10. Isn't that a stretch for 1/6, 1/7, 1/8?

manjot123 · Answer

take extreme cases 
5/6 is smallest so 
take all numbers be 5/6 it will give smallest value 
6*5/6 = 5
largest = 6*10/11
so 5<n<60/11

bumpbot · Answer

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If N = 5/6 + 6/7 + 7/8 + 8/9 + 9/10 + 10/11, then which of the followi

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If N = 5/6 + 6/7 + 7/8 + 8/9 + 9/10 + 10/11, then which of the followi

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