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

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

@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.

The $n^{th}$ term of this series is $A_{n}=A_{n-1}-n$ where $25$ is the $0th$ term; hence the $5^{th}$ term which is what your looking for is $A_{5}=A_{4}-5$. Therefore $A_{5}=15-5$ which is $10$ hence the answer is $C$.

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This discussion board is a place to discuss our Daily Challenges and the math and science related to those challenges. Explanations are more than just a solution — they should explain the steps and thinking strategies that you used to obtain the solution. Comments should further the discussion of math and science.

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## Comments

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TopNewestAlthough the intended answer here is $C$, you should know that there is no

correctanswer to this. I could easily say the answer is $B$.You can ask me why. Well, my explanation is: the $n ^{th}$ term of this sequence is $f(n)$ where $f(n)=-\frac{n^5-15n^4+85n^3-213n^2+262n-720}{24}.$Plug in $n=1, 2, 3, 4, 5$ and get $f(n)= 25, 24, 22, 19, 15$ [your sequence]. Plug in $n=6$ and you'll get $f(n)=5$. So, $B$ 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

correctanswer.Alt text

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How did u get this f(n)

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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

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

For example: you could add $(n-1)(n-2)(n-3)(n-4)(n-5)$ 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!

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The $n^{th}$ term of this series is $A_{n}=A_{n-1}-n$ where $25$ is the $0th$ term; hence the $5^{th}$ term which is what your looking for is $A_{5}=A_{4}-5$. Therefore $A_{5}=15-5$ which is $10$ hence the answer is $C$.

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

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10

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10

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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)

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C,10

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

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10

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c

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It's C. 10

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