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

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

Now observe the pattern in the differences ($21,27,33$):

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

The differences are increasing by a constant value of $6$ each time. Following this logic, the next difference must be:

$$33+6=39$$

Therefore, the next number in the series will be:

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