Question

Students are asked to memorize a list of 100 words. The students are given periodic quizzes to see how many words they have memorized. The function L gives the number of words memorized at time t. The rate of change of the number of words memorized is proportional to the number of words left to be memorized.

1. Which of the following differential equations could be used to model this situation, where k is a positive constant?
A. dL/dt = kL
B. dL/dt = 100 - kL
C. dL/dt = k(100 - L)
D. dL/dt = kL - 100

Answer 1

C. dL/dt = k(100 - L)

Explanation:

We have a list containing 100 words.

L = number of words memorized at time t.

At any time t, the number of words left to be memorized is 100-L.

Therefore:

`(dL)/(dt)\propto 100-L\\ $Introducing k, a constant of proportion$\\(dL)/(dt)= k(100-L)`

The correct option is C.