The Average Typing Speed N (In Words Per Minute) After $N = \frac { 91 } { 1 + 7.5 e ^ { - 0.15 t } }$

Multiple Choice

The average typing speed N (in words per minute) after t weeks of lessons is modeled by $N = \frac { 91 } { 1 + 7.5 e ^ { - 0.15 t } }$ . Find the rate at which the typing speed is changing when t = 20 weeks. Round your answer to two decimal places.

A) 2.40 words/min/week
B) 2.70 words/min/week
C) 3.71 words/min/week
D) 4.93 words/min/week
E) 6.24 words/min/week