Question of the Week - 3 (The value of the variable E is...)

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The value of the variable E is determined by adding the reciprocals of the first 10 even natural numbers. Which of the following can be a possible value of the reciprocal of E?
  • A. 0.154
    B. 0.1818
    C. 0.667
    D. 2
    E. 3.03


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by [email protected] » Sun Apr 07, 2019 5:39 am
e-GMAT wrote:The value of the variable E is determined by adding the reciprocals of the first 10 even natural numbers. Which of the following can be a possible value of the reciprocal of E?
  • A. 0.154
    B. 0.1818
    C. 0.667
    D. 2
    E. 3.03
A fast approach is to use some estimation.

E = 1/2 + 1/4 + 1/6 + 1/8 + 1/10 + 1/12 + 1/14 + 1/16 + 1/18 + 1/20
≈ 0.5 + 0.25 + 0.15 + 0.1 + 0.1 + 0.1 + 0.05 + 0.05 + 0.05 + 0.05
≈1.4

Which of the following can be a possible value of the reciprocal of E?
The reciprocal of E ≈1/1.4 ≈0.7

Check the answer choices . . . answer = C

Cheers,
Brent
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by e-GMAT » Wed Apr 10, 2019 9:26 pm
Solution

Given:
In this question, we are given that
  • "¢ The value of the variable E is determined by adding the reciprocals of the first 10 even natural numbers.
To find:
We need to determine
  • "¢ Among the given options, which one can be a possible value of the reciprocal of E.
Approach and Working:
As E equals the sum of the reciprocals of the first 10 even natural numbers, we can write E in the following manner:
  • "¢ E = 1/2 + 1/4 + 1/6 + 1/8 + 1/10 + 1/12 + 1/14 + 1/16 + 1/18 + 1/20
    Or, E = 1/2 (1 + 1/2 + 1/3 + 1/4 + 1/5 + 1/6 + 1/7 + 1/8 + 1/9 + 1/10)
Now, if we observe the terms within the bracket carefully, we can see that 1 is the largest among all the 10 terms and 1/10 is the smallest among all the 10 terms.
  • "¢ If all 10 terms were equal to 1/10, then E would be 1/2 x 10 x 1/10 = 1/2.
    • o But since the actual terms are more, we can say that the value of E is greater than 1/2.
    "¢ Similarly, if all 10 terms were equal to 1, then E would be 1/2 x 10 x 1 = 5.
    • o But since the actual terms are less, we can say that the value of E is less than 5.
Combining the above results, we can say
  • "¢ 1/2 < E < 5
    Or, 2 > 1/E > 1/5
    Or, 2 > 1/E > 0.2
From the given options, only 0.667 falls in the given range.
Hence, the correct answer is option C.

Answer: C