The answer is $84.29
Let the number of minutes be x.
$35.99 is fixed price and does not depend on the number of minutes x. Each minute costs $0.21 so it depends on x and we have 0.21x in the function. However, 250 minutes are already included and should not be included in x. Therefore, the function that represents the monthly cost looks like this:
f(x) = 35.99 + 0.21 * (x - 250)
Now, let's calculate for x = 480 minutes:
f(480) = 35.99 + 0.21 * (480 - 250)
f(480) = 35.99 + 0.21 * 230
f(480) = 35.99 + 48.30
f(480) = 84.29
