Converting a Recurrence Relation to its Closed Form
It might be helpful if N is assigned a range of values from 1 to say 7 and evaluated till the recursion end is reached.
also summing together terms with 15 multiples separately and 40 multiple separately, because they are the building blocks from the base case and recursion. 

T(1) = 40.

T(2) = 40.

....


T(6) = 2 * ( 2 * 40 - 15 ) - 15 
           = 4 * 40 - 3 * 15 

T(7) = 2 * ( 2 * ( 2 * 40 -  15 ) -15 ) - 15
           = 8 * 40 - 7 * 15

i think this can help to find the closed form. But i am not an expert in this area.