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Bunuel, I'm having difficulty understanding this question and the solution. Can you please help?
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Could we do an aritmetic progression here?
Something like a19 = a1 + (n + 1) * d
a19 = 4 + (19+1) * 2
a19 = 4 + 40
a19 = 44
Why wouldn´t this work? (1/44 on week 19th)
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ruis
Could we do an aritmetic progression here?
Something like a19 = a1 + (n + 1) * d
a19 = 4 + (19+1) * 2
a19 = 4 + 40
a19 = 44
Why wouldn´t this work? (1/44 on week 19th)

This would not work for the simple reason that the given sequence is not an arithmetic progression; it is a geometric progression.
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Week_19 ratio is 1/2^20

Knowing from basics of binary conversion 2^10=1024
2^10>10^3
1/2^10<1/10^3
1/2^20<1/10^6

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Denomenator doubles every week so geometric formula would be 1/2^20. It is such a small number you don't need to solve. It has to be smaller than 1/1,000,000. IMO this question tests your ability to recognize the "exponentialness" of exponents.­
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Last year a company produced millions of widgets each week. Last year the ratio of the number of defective widgets to the number of widgets produced was \(\frac{1}{4}\) for the first week, \(\frac{1}{8}\) for the second week, \(\frac{1}{16}\) for the third week, and so on for 19 weeks, where the ratio for each week after the first week was half of the ratio for the preceding week. If last year the ratio of the number of defective widgets to the number of widgets produced was \(d\) for the 19th week, then \(d\) satisfies which of the following inequalities?

A. \(d < \frac{1}{1,000,000}\)

B. \( \frac{1}{1,000,000} \leq d < \frac{1}{100,000} \)

C. \( \frac{1}{100,000} \leq d < \frac{1}{10,000} \)

D. \( \frac{1}{10,000} \leq d < \frac{1}{1,000} \)

E. \(d \geq \frac{1}{1,000}\)

Attachment:
Screenshot 2023-12-07 130523.png
­

The question gives a GP.

\(1st Term = \frac{1}{4} = \frac{1}{2^2}\)
\(2nd  Term = \frac{1}{8} = \frac{1}{2^3}\)
...
\(19th Term = \frac{1}{2^{20}} = d\)

\(d = \frac{1}{1024*1024} \)

So d is less than \(\frac{1}{1000*1000} = \frac{1}{1,000,000}\)

Answer (A)

Check this post on GPs
https://anaprep.com/algebra-benefits-of ... -concepts/­
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Given: Last year a company produced millions of widgets each week. Last year the ratio of the number of defective widgets to the number of widgets produced was \(\frac{1}{4}\) for the first week, \(\frac{1}{8}\) for the second week, \(\frac{1}{16}\) for the third week, and so on for 19 weeks, where the ratio for each week after the first week was half of the ratio for the preceding week.

Asked: If last year the ratio of the number of defective widgets to the number of widgets produced was \(d\) for the 19th week, then \(d\) satisfies which of the following inequalities?

\(d = 1/2^{n+1} = 1/2^{20} = 1/{1024}^2 < 1/10^6\)

IMO A
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