Complete the series:

Question

$4427, 827, 67, 31, 9, ?$

Choose between:

• (a): $7$
• (b): $6$
• (c): $5$
• (d): $4$

Looking at the numbers I think that there must be some exponential relationship but couldn't find it. I took the sequence of differences between the numbers which is $3600, 760, 36, 22, ?$
But I still couldn't find any relationship.

I do not think the question is wrong.

Source : NTSE Test Series, MAT

Answer 1

The answer maybe:

(a): 7

The reason is:

4427 = 672 - 2 × 31
 827 = 312 - 2 × 67

  67 =  92 - 2 ×  7
  31 =  72 - 2 ×  9

I don't think this is just a coincidence :)