Answer
4th TV: 20 minutes
Total: 118 minutes
Work Step by Step
We find how long it takes to assemble the fourth one:
$$ = (40)(.8)(.8)(.8) \\ = 20 \ minutes$$
The total amount of time needed to assemble all four is:
$$ =40 +(.8)(40)+ (.8)(.8)(40) + (.8^3)(40) \\ =118 \ minutes$$
