00:01
This problem says the credit remaining on a phone card in dollars is a linear function of the total calling time made with the card in minutes.
00:08
The remaining credit card after 44 minutes of calls is $18 .84, and the remaining credit after 67 minutes of calls is $15 .62.
00:18
What's the remaining credit after 72 minutes of calls? and before we worry about what the remaining credit would be after 72 minutes, we're going to come up with a linear function to represent this scenario by treating our first two faxes as ordered pairs.
00:29
So we'll show 44 minutes giving us the credit of $18 .84 as x1, y1, and then 67 minutes giving us $15 .62 as credit as x2, y2.
00:43
And for our linear function or linear equation, we'll write it in the form y equals mx plus b, and we'll find our slope first, or our m value, the slope formula of y2 minus y1 over x2 minus x1.
00:56
So that would give us y2 of $15 .62 minus $18 .84 over x2 of 67 minus x1 of 44.
01:08
And when we subtract, we get negative 3 .22 divided by 67 minus 44, and 67 minus 44 is going to give us 23.
01:19
And when we divide, that gives us negative 0 .14.
01:22
So that's our slope...