What should come in the place of question mark in the following series?
$3,8,6,14,?,20$

The given arrangement functions as an alternating series containing two completely independent arithmetic progressions merged together:
* First progression (odd-numbered positions): $3,6,\dots$
The sequence advances consistently by adding $+3$:

$$3\stackrel{+3}{\to }6\stackrel{+3}{\to }9$$

* Second progression (even-numbered positions): $8,14,20$
The sequence advances consistently by adding $+6$:

$$8\stackrel{+6}{\to }14\stackrel{+6}{\to }20$$

Thus, the missing number at the 5th position is $9$.