[8 pts] The average typing speed N (in words per minute) after t weeks of lessons is modeled by:

N=95/(1+8.5e^(-0.12t) )

a. Find the rate at which the typing speed is changing after 10 weeks of lessons.

b. Find the rate at which the typing speed is changing after 20 weeks of lessons.

c. How long do you think the lessons should last? Explain your reasoning.