arithmetic

Which number should come next in this series?

25,24,22,19,15

A. 4 B. 5 C. 10 D. 14

Note by Pryhant Kielh
6 years, 2 months ago

No vote yet
4 votes

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Although the intended answer here is CC, you should know that there is no correct answer to this. I could easily say the answer is BB.

You can ask me why. Well, my explanation is: the nthn ^{th} term of this sequence is f(n)f(n) where f(n)=n515n4+85n3213n2+262n72024.f(n)=-\frac{n^5-15n^4+85n^3-213n^2+262n-720}{24}.Plug in n=1,2,3,4,5n=1, 2, 3, 4, 5 and get f(n)=25,24,22,19,15f(n)= 25, 24, 22, 19, 15 [your sequence]. Plug in n=6n=6 and you'll get f(n)=5f(n)=5. So, BB is a perfectly valid answer.

I know that this is not the answer you want and you're probably getting angry at me right now! What I'm trying to say is there's no correct answer.

Alt text Alt text

Mursalin Habib - 6 years, 2 months ago

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How did u get this f(n)

Tushar gautam - 6 years, 1 month ago

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why was that comment deleted i wrote in it that tushar is a cheater who cheats from many id's and has atleast 20 ids on brillaint

Iitian Singh - 6 years, 1 month ago

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@Iitian Singh Tushar's many accounts have been banned. Please in the future email me at discussions@brilliant.org to report cheating. I deleted the comment because, I thought it distracted from the point Mursalin was trying to make.

Peter Taylor Staff - 6 years, 1 month ago

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I used Lagrange interpolation. But this is not the only way.

For example: you could add (n1)(n2)(n3)(n4)(n5)(n-1)(n-2)(n-3)(n-4)(n-5) to the polynomial to get a different value for the sixth term.

This way you can practically get anything (even complex numbers) for the sixth term!

Mursalin Habib - 6 years, 1 month ago

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The nthn^{th} term of this series is An=An1nA_{n}=A_{n-1}-n where 2525 is the 0th0th term; hence the 5th5^{th} term which is what your looking for is A5=A45A_{5}=A_{4}-5. Therefore A5=155A_{5}=15-5 which is 1010 hence the answer is CC.

Joel Jablonski - 6 years, 2 months ago

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10, because 25-1=24, 24-2=22,22-3=19,19-4=15, so 15-5=10

tiara yosephine - 5 years ago

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10

Rahul Sharma - 4 years, 9 months ago

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10

Abu Taher - 4 years, 2 months ago

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we can do this in this way to: here consecutive nos are being subtracted.i from 25 then 2 from 24 and then 3 from 22 and so on..........

so the ultimate answer is 15-5(as it is the 5th term in sequence)=10 (c)

yash gupta - 6 years, 1 month ago

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C,10

Nasir Mehmood Alam - 6 years, 1 month ago

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Well, It is necessary to ask you what part of Mathematics is it after we will conclude an answer.

Jeffer Dave Cagubcob - 6 years, 1 month ago

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10

Anubhav Singh - 6 years, 1 month ago

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c

mounika pokala - 6 years, 1 month ago

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It's C. 10

Brij Rokad - 6 years, 2 months ago

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