$\"\"$

A cell phone plan has a basic charge $35 per a month.

This plan has a condition that, one must have to use less than or equal to 400 minutes in a month.

$\"\"$ be the number of minute used in a month and $\"\"$ be the total monthly charges.

The usage charges for 400 minutes or less than 400, then the function is $\"\"$ .

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Find the cost function if user the uses more than 400 minutes :

Consider above 400 minutes are the additional minutes.

For every additional minute, it charges a 10 cents per minutes.

The number of minutes in a month = 400 minutes + additional minutes.

Then the additional minute are $\"\"$ .

The charges of the additional minutes are $\"\"$ .

The total monthly charges = basic charges + additional charges.

$\"\"$

The function is $\"\"$ .

So the cost function is $\"\"$ .

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Graph :

Graph of the function $\"\"$ in the interval $\"\"$ .

$\"\"$ axis : monthly cost $\"\"$ .

$\"\"$ axis : number of minutes usage $\"\"$ .

$\"\"$

The function is $\"\"$ .

Graph of the function $\"\"$ in the interval $\"\"$ .

$\"\"$ .