Verbal Reasoning - Series Completion - Discussion
Discussion Forum : Series Completion - Section 1 (Q.No. 15)
Directions to Solve
Choose the correct alternative that will continue the same pattern and replace the question mark in the given series.
15.
Which term of the series 5, 10, 20, 40, ..... is 1280?
Answer: Option
Explanation:
Clearly, 5 x 2 = 10, 10 x 2 = 20, 20 x 2 = 40,.....
So, the series is a G.P. in which a = 5 and r = 2.
Let 1280 be the rath term of the series.
Then, 5x2n-1 = 1280 
2n-1 = 256 = 28 n - 1 = 8 n = 9.
Discussion:
5 comments Page 1 of 1.
Ankit.P.Vyas said:
1 decade ago
If the above mentioned series is solved as follows:-
5*2=>10
10*2=>20
20*2=>40
40*2=>80
80*2=>160
160*2=>320
320*2=>640
640*2=>1280
Then it would be the 8th term of these series :- 5, 10, 20, 40,.... is 1280.
Please do the necessary changes if any, and if i am taking the series in the wrong way, can you please explain me your answer.
Awaiting for your reply...
5*2=>10
10*2=>20
20*2=>40
40*2=>80
80*2=>160
160*2=>320
320*2=>640
640*2=>1280
Then it would be the 8th term of these series :- 5, 10, 20, 40,.... is 1280.
Please do the necessary changes if any, and if i am taking the series in the wrong way, can you please explain me your answer.
Awaiting for your reply...
Khurram said:
1 decade ago
@Ankit: You're forgetting the first term: 5.
Add 5 to the above table you've made and it then counts to 9th term. Do it your way or theirs, it remains 9th.
Add 5 to the above table you've made and it then counts to 9th term. Do it your way or theirs, it remains 9th.
Shreedevi said:
1 decade ago
Please tell me the correct answer 5*2=10,
10*2=20, 20*2=40, 40*2=80, 80*2=160, 160*2=320, 320*2=640, 640*2=1280 So the answer is 8th term how it could be 9th?
10*2=20, 20*2=40, 40*2=80, 80*2=160, 160*2=320, 320*2=640, 640*2=1280 So the answer is 8th term how it could be 9th?
Karan said:
1 decade ago
5 is first.
10 is second.
20 is third.
40 is fourth.
80 is fifth.
160 is sixth.
320 is seventh.
640 is eight.
1280 is ninth.
So 1280 is ninth term.
10 is second.
20 is third.
40 is fourth.
80 is fifth.
160 is sixth.
320 is seventh.
640 is eight.
1280 is ninth.
So 1280 is ninth term.
(3)
Jiji said:
2 years ago
Gp formula is 1(2^n-1)/(2-1). Then, how it become 2^(n-1)?
Please explain.
Please explain.
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