$\"\"$

The cost function is $\"\"$ .

For the first $\"\"$ minutes the phone service charges $\"\"$ .

The cost function for first $\"\"$ minutes is $\"\"$ .

For each additional minute or fraction the phone service charge is increased $\"\"$ .

If the time $\"\"$ is not integer than cost function is $\"\"$ .

If the time $\"\"$ is an integer than cost function is $\"\"$ .

The cost function is $\"\"$

Graph the function:

$\"\"$

Observe the graph:

The non-removable discontinuity at each integer greater than or equal to $\"\"$ .

$\"\"$

The cost function is $\"\"$

Graph:

$\"\"$