SOLUTION: The credit remaining on a phone card (in dollars) is a linear function of the total calling time made with the card (in minutes). The remaining credit after 28 minutes of calls is
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Question 1111736: The credit remaining on a phone card (in dollars) is a linear function of the total calling time made with the card (in minutes). The remaining credit after 28 minutes of calls is $16.64, and the remaining credit after 65 minutes of calls is $12.20. What is the remaining credit after 89 minutes of calls?
You have a linear function with the values of
= 16.64 at x = 28, and
= 12.20 at x = 65.
Hence, the slope of the straight line is
m = = = -0.12.
// Surely, you do understand that |m| = |-0.12| = 0.12 dollars is the cost rate (= price) for one minute of the phone conversation.
Now the value of "y" at x= 89 minutes is
y = + = 12.20 -0.12*(89-65) = 9.32 dollars.
Answer. The remaining credit after 89 minutes of calls is 9.32 dollars.
