c = 8t + 12 (option C)
Explanation:
The equation will be in the form of equation of line:
y = mx + c
where y = cost
m = slope, x = change for calls, c = intercept
m = change for calls/change in lenght of call
m = slope = (28-20)/(2-1) = (36 -28)/(3-2)
m = 8/1 = 8
So we know the rate of change is 8
for t = 1, = 8 *1 = 8
for t = t , 8*t =8t
The option in the question with 8t is c = 8t + 12
To confirm if the option is correct, we use any of the value of time:
when t = 2
c = 8(2) + 12 = 16 + 12
c = $28
This is correct from the table. Hence, the equation can be used to determine the cost c is c = 8t + 12 (option C)