# Find the term which does not fit into the series given below

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?

• So the goal is to find one that doesn't fit? So one of the numbers is in fact not part of the series? Oct 14 '15 at 6:27
• Yup. @DrunkWolf. Oct 14 '15 at 6:28

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

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