The monthly cost for 64 minutes of calls is $ 20.94
Solution:
Given, The monthly cost (in dollars) of a long-distance phone plan is a linear function of the total calling time (in minutes).
Let it be, m = an + b
Where, m is monthly cost, a is cost per minute, n is number of minutes of call, and b is initial charges.
The monthly cost for 47 minutes of calls is $18.90
Then, 18.9 = 47a + b ⇒ (1)
And the monthly cost for 83 minutes is $23.22.
Then, 23.22 = 83a + b ⇒ (2)
Now, subtract (1) from (2)
36a = 4.32
a = 0.12
Now substitute a value in (1)
47(0.12) + b = 18.9
b = 18.9 – 5.64
b = 13.26
Then, our equation becomes m = 0.12n + 13.26 --- eqn (3)
We have to find what is the monthly cost for 64 minutes of calls?
So ,substitute n = 64 in (3)
m = 0.12 x 64 + 13.26
m = 7.68 + 13.26 = 20.94
Hence, the cost is $ 20.94 for 64 minutes of call
