Find the missing number in the given series.
11, 15, 24, 40, 65, ?

Let's find the difference between consecutive terms:
* $15-11=4=2^2$
* $24-15=9=3^2$
* $40-24=16=4^2$
* $65-40=25=5^2$

The differences form a sequence of consecutive perfect squares ($2^2,3^2,4^2,5^2$).
The next difference must be $6^2=36$.
Missing number = $65+36=101$.