# Number Sequence Series-Question 4

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?

## 3 Answers

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.

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

Is it:

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

• Sorry, it is not correct. Pls check the right answer. – LETTERKING May 19 at 10:37

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 = $$\frac{21870}{81} = 270$$

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