Find the next number in the series $15,36,63,96,\_\_\_\_\_\_\_$.

Let's find the difference between the consecutive terms in the series:
* $36-15=21$
* $63-36=27$
* $96-63=33$

Notice that the differences ($21,27,33$) form an arithmetic progression increasing by $+6$ at each step:

* $27-21=6$
* $33-27=6$

Following this established pattern, the next difference must be:

$$33+6=39$$

Therefore, the next number in the series is:

$$\text{Next Number}=96+39=135$$