In the series F, J, N, R, _ which letter comes next?

Let's map the letters to their numerical positions in the alphabet ($A=1,B=2,\dots$):
* $\text{F}=6$
* $\text{J}=10$
* $\text{N}=14$
* $\text{R}=18$

The pattern shows a constant addition of $+4$ to the previous position:

* $6+4=10$
* $10+4=14$
* $14+4=18$

Following the same logic, the next position will be:

$$18+4=22$$

The $22^{\text{nd}}$ letter of the alphabet is $\text{V}$.