Find the missing numbers $(A,B)$ in the series and find the value of $B\times A$.
$$26,39,52,A,78,91,B$$

Let's look at the common difference between successive terms:
* $39-26=13$
* $52-39=13$

The series progresses by adding $13$ each time (multiples of 13 starting from $13\times 2$):

* $A=52+13=65$
* Check: $65+13=78$
* $78+13=91$
* $B=91+13=104$

Now, we need to find the value of $B\times A$:

$$B\times A=104\times 65=6760$$