C4T, F10R, I20P, L43N, O90L

It can be easily seen that the first letters are 3rd, 6th, 9th, 12th, 15th and the last ones are 20th, 18th, 16th, 14th, 12th. But I cannot understand the logic of numbers between them. So please can someone explain it?

  • $\begingroup$ So the goal is to find one that doesn't fit? So one of the numbers is in fact not part of the series? $\endgroup$
    – DrunkWolf
    Oct 14, 2015 at 6:27
  • $\begingroup$ Yup. @DrunkWolf. $\endgroup$ Oct 14, 2015 at 6:28

2 Answers 2


I think odd one is F10R. It should have been F9R.

All the numbers will follow a pattern if it's F9R.

Like the first number is 4. So second is 4+4 + 1 = 9.. Next

9+9 + 2 =20..

20+20 + 3=43..

43+43 + 4=90..

I am not sure though


The number sequence is defined by

$x_n = 3\cdot2^n-n-1$

This yields

$x_1 = 3\cdot2^1-1-1=4$
$x_2 = 3\cdot2^2-2-1=9$
$x_3 = 3\cdot2^3-3-1=20$
$x_4 = 3\cdot2^4-4-1=43$
$x_5 = 3\cdot2^5-5-1=90$
$x_6 = 3\cdot2^6-6-1=185$


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