-4
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Find out the next number in following Simple series,

15, 55, 120, 210, 325, ?, ?

Note: Question has many solutions.

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  • $\begingroup$ It's 5 times 3,11,24,42,65,... $\endgroup$ – Rand al'Thor May 9 '15 at 10:43
  • $\begingroup$ explain more! what's this series 3,11,24,42,65.... $\endgroup$ – Saurabh Prajapati May 9 '15 at 10:46
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    $\begingroup$ A lot of answers, and not the simple one: it's the sequence containing every 5th triangle number. $\endgroup$ – Phylogenesis May 9 '15 at 13:00
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    $\begingroup$ You shouldn't add a solution to your question. There's a button at the bottom to "answer your own question" if this is what you want to do. $\endgroup$ – Rand al'Thor May 9 '15 at 13:01
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    $\begingroup$ OEIS A144312 $\endgroup$ – Tryth May 9 '15 at 13:07
5
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Answer is:

465

Because:

You add 5 to the factor of 5 being added. Between first two you add $5 \times 8$, next one you add $5 \times 13$, then $5 \times 18$, then $5 \times 23$, so next you add $5 \times 28$, ending with $5 \times 93 = 465$.

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3
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It's

$$465$$

Because:

$$t_n=\frac{5}2n(n+5)$$

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  • $\begingroup$ Are you sure??? $\endgroup$ – Saurabh Prajapati May 9 '15 at 12:08
  • $\begingroup$ @SaurabhPrajapati yes. $\endgroup$ – RE60K May 9 '15 at 15:52
  • $\begingroup$ @SaurabhPrajapati In which way my answer is different or wrong than the accepted answer. $\endgroup$ – RE60K May 13 '15 at 4:52
1
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You start by (adding) 15, so you have 15. From then you keep adding the same number as before plus 25.

Therefore the sequence is:

$$15,55,120,210,325,465,630$$

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  • $\begingroup$ sorry , but i can't understand? please explain more.. $\endgroup$ – Saurabh Prajapati May 9 '15 at 12:37
  • $\begingroup$ You first add 15 (from 0), then 15+25=40, then 40+25=65, then 65+25=90, and so on. $\endgroup$ – Masclins May 9 '15 at 12:38
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    $\begingroup$ your answer and logic both are Right! $\endgroup$ – Saurabh Prajapati May 9 '15 at 12:57

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