Answer:

Explanation:
This problem can be modeled by simple decreasing first order function.
This function has the following format

In which R(t) is the remaining time, in minutes,
is the initial remaining time, and a is the angular coefficient of the function.
In your problem, the prepaid phone card has a credit of 200 minutes, so
. At each minute, the amount of time remaining will decrease by 1, so the angular coefficient is 1, a = 1.
We are left with:
