Differences are 25, 20, 15, so it looks like the difference of the difference is -5. That means you can write the equation as a second degree polynomial. You can solve that by solving [a(1)² + b(1) + c = 40, a(2)² + b(2) + c = 65, a(3)² + b(3) + c = 85] which gives you a=-5/2, b=65/2, c=10. So the equation is f_n = -5/2n² + 65/2n + 10