Question

Dependent variables can be the result of more than one independent variable. Consider your phone bill as presented in the Opening Activity. Aside from being charged for minutes over 300, you are charged $1.25 for each directory assistance call and $.75 per minute for long distance roaming calls. You also have to contribute at least $1.50 to the federally required Universal Service Fund per month and this value may vary on your phone bill.

Month
Fixed monthly fee
($35.00 per month)
Additional minutes used
($.40 per min.)
Directory assistance
($1.25 per call)
Long distance roaming minutes
($.75 per minute)

USF monthly charge Total Monthly Bill
January

55
2
0 1.50
February

14
1
15 1.50
March

0
3
0 2.25
April

86
4
35 1.50
May

75
2
15 2.50
June

0
0
0 1.50
What is the constant?

How many independent variables are there in this situation?

What are the independent variable(s)? What independent variable(s) do you have control over and what independent variable(s) do you not have control over?

What is the dependent variable?

Write an equation expressing your monthly cost using the below independent and dependent variables.
Let x = additional minutes used
Let d = number of directory assistance calls made
Let r = number of long distance minutes used
Let u = USF monthly charge
Let y = total monthly bill

Using your equation, fill in the Total Monthly Bill column in the above table.

You can upgrade your service to $40 a month for 500 anytime minutes? Which independent variable would you consider to determine if this is in your best interest? Do you think you should upgrade?