Find the next number in the following series:
$$140,120,90,50,\underline{}$$

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

The difference between consecutive terms increases by $-10$ each time (i.e., subtracting $20,30,40,\dots$).
Following this pattern, the next number is obtained by subtracting $50$:

$$\text{Next number}=50-50=0$$