Given that the monthly cell phone bill is determined by a fixed monthly fee of $35
for 300 anytime minutes and $.40 for each additional minute used.
Dependent variables can be the result of more than one independent
variable.
Consider your phone bill as presented above. 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 but is not an independent variable.
To write
an equation expressing your monthly cost using the below independent
and dependent variables.
Let x = additional minutes used, then the cost for additional minutes used is given by 0.4x.
Let d = number
of directory assistance calls made, then the cost for directory assisitance is given by 1.25d.
Let r = number of long distance
minutes used, then the cost for long distance minutes is given by 0.75r.
Let u = USF monthly charge.
Let y = total monthly bill, therefore the total monthly bill is given by
y = 0.4x + 1.25d + 0.75r + u
