Find the next number in the following series:
140, 120, 90, 50, __

Let's find the difference between consecutive numbers:
* $120-140=-20$
* $90-120=-30$
* $50-90=-40$

The differences are decreasing by an additional $10$ each time (i.e., $-20,-30,-40,\dots$).
Thus, the next difference must be $-50$.

The next number is:

$$50-50=0$$