In the series $\text{F, J, N, R, \_\_\_}$ which letter comes next?

Let's convert the alphabet positions into numerical positions ($\text{A}=1,\text{B}=2,\dots ,\text{Z}=26$):
* $\text{F}=6$
* $\text{J}=10$
* $\text{N}=14$
* $\text{R}=18$

Let's check the differences between consecutive terms:

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

The pattern is adding $4$ to each successive term. Following the same pattern, the next number will be:

$$18+4=22$$

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