Question 1127039
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A linear function is of the form y = ax+b.<br>
When x is 0, the function value is b.  On a graph, b is the y-intercept of the function.<br>
When x increases by some amount, the function value increases by "a" times that amount.  a is the slope of the line; it tells how fast the function value increases for each increase in x.<br>
In your problem...
the function y is the cost of the call
the price increases as the number of minutes increases; the slope "a" tell you how much the cost increases for each additional minute
b tells you how much the call would cost if the call lasted 0 minutes; it is the "base" cost of the call.<br>
To solve the problem, do the following:<br>
(1) Find the slope, a.  You know how much the cost increases for the 6 additional minutes; find the increase in cost for each minute.
(2) Using the $2.25 cost of the 6-minute call, and the slope you found in (1) above, determine what the cost would be for 0 minutes (subtract 6 times the cost-per-minute charge from the total cost of the 6-minute call).  That will be the b in your linear function.<br>
Now you have your a and b values, so you can write the function.<br>
Now the second part is easy; simply evaluate your function for x = 20.  (Or, if you understand what you are doing, you could find the cost of the 20-minute call by adding 8 times the cost-per-minute charge to the given cost of the 12-minute call.)