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 40 minutes of calls is $25.20, and the remaining credit after 72 minutes of calls is $21.36. what is the remaining credit after 86 minutes of calls?

(40,25.20)(72,21.36)
slope = (21.36 - 25.20) / (72 - 40) = -3.84 / 32 = -0.12

y = mx + b
slope(m) = -0.12
(40,25.20)...x = 40 and y = 25.20
sub and solve for b, the y int
25.20 = -0.12(40) + b
25.20 = - 4.8 + b
25.20 + 4.8 = b
30 = b

ur equation is : y = -0.12x + 30where x is ur minutes, and y is remaining credit.

after 86 minutes...
y = -0.12(86) + 30
y = - 10.32 + 30
y = 19.68 <===