Question

A prepaid phone card has a credit of 200 minutes. Write an expression for the number of minutes left on the card after t minutes have been used.

Answer 1

`R(t) = 200 - *t`

Explanation:

This problem can be modeled by simple decreasing first order function.

This function has the following format

`R(t) = R_(0) - a*t`

In which R(t) is the remaining time, in minutes,
`R_(0)` 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
`R_(0) = 200`. At each minute, the amount of time remaining will decrease by 1, so the angular coefficient is 1, a = 1.

We are left with:

`R(t) = 200 - *t`