The value of f(320) = $78.2.
And the meaning of f(320) is the the amount $ 78.2 Derek pays for 320 minutes of international calls.
Given the function: f(x) = 0.11x + 43 ……
x represents the number of minutes used for international calls in a month and
f(x) represents the cost.
Find the value of f(320);
Substitute the value of x =320 in  we have;
Therefore, the value of f(320) is, $78.2.
The meaning of f(320) means that $78.2 is the amount Derek pays for 320 minutes of international calls.