# What's the Next number in series #3? [closed]

Find out the next number in following Simple series,

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

Note: Question has many solutions.

• It's 5 times 3,11,24,42,65,... – Rand al'Thor May 9 '15 at 10:43
• explain more! what's this series 3,11,24,42,65.... – Saurabh Prajapati May 9 '15 at 10:46
• A lot of answers, and not the simple one: it's the sequence containing every 5th triangle number. – Phylogenesis May 9 '15 at 13:00
• 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. – Rand al'Thor May 9 '15 at 13:01
• OEIS A144312 – Tryth May 9 '15 at 13:07

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 $5 \times 93 = 465$.

It's

$$465$$

Because:

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

• Are you sure??? – Saurabh Prajapati May 9 '15 at 12:08
• @SaurabhPrajapati yes. – RE60K May 9 '15 at 15:52
• @SaurabhPrajapati In which way my answer is different or wrong than the accepted answer. – RE60K May 13 '15 at 4:52

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:

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

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