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I want to share a series of questions that are created by myself.

I will give a hint in 24 hours and my answer in 3 days if nobody could answer my question.

Previous ones are here:

Number Sequence Series-Question 1

Number Sequence Series-Question 2

Number Sequence Series-Question 3

Here is the new number sequence:

21870,1215,?,15

If you guys want some extremely challenging questions. Please check these two questions posted by me.

Number sequences: 000, X00... and 6X000X9, 700XX08

What are the alphabets in the question mark?

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The answer is

135.

You can generate this by

dividing by the sum of the digits to get the next number. For instance the sum of digits of 21870 is 2+1+8+7+0=18, so divide by 18 to get 1215, whose digit sum is 1+2+1+5=9, so we divide by 9 to get 135, and divide by 9 again to get 15 as desired.

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  • $\begingroup$ Yes it is correct. Congrats. $\endgroup$ – LETTERKING May 19 '19 at 10:34
  • $\begingroup$ As I said, the logic of a good number sequence question must be the neatest and it comes from the question ITSELF. $\endgroup$ – LETTERKING May 19 '19 at 10:35
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Is it:

$135$. Remove all factors of $2$ and divide by $9$.

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  • $\begingroup$ Sorry, it is not correct. Pls check the right answer. $\endgroup$ – LETTERKING May 19 '19 at 10:37
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I think it is,

270.

Reason

The ratio of first and second number of a pair yields 18. $$$$ Also, we can find that the ratio of second and fourth number is 81, so the ratio of first and third numbers should also be 81. $$$$ $\frac{21870}{1215} = 18$ So, third term = $15\cdot 18 = 270$ $$$$ Also, $\frac{1215}{15} = 81$ so, third term = $\frac{21870}{81} = 270$

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  • $\begingroup$ Sorry, it is not correct. Pls check the right answer. $\endgroup$ – LETTERKING May 19 '19 at 10:37
  • $\begingroup$ Ok thanks, and when are you posting your next question? $\endgroup$ – Ak19 May 19 '19 at 10:41
  • $\begingroup$ You can try my Number Sequence 3 and I will post my fifth question in 3 hours. $\endgroup$ – LETTERKING May 19 '19 at 10:43

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