69,017,409 is the correct answer
T = k/R where k is the constant of variation, T=time and R=speed.
Plug in the given values:-
12 = k/60
k = 12*60 = 720
So the equation of variation is T = 720/R
At 85 mph we have
T = 720 / 85 = 8.47 hours (answer)

As

, the sequence

converges to zero.
If you're talking about the infinite series

well we've shown by comparison that this series must also converge because we know any geometric series

will converge as long as

.
Step-by-step explanation:
81x⁴ - 16y⁴
(9x²)² - (4y²)²
(9x² + 4y²)(9x² - 4y²)
(9x² + 4y²)(3x + 2y)(3x - 2y)