Question 941566
Let y be Credit remaining  and t be the time 
  assume y = at+b
 where a, be are constants
when t= 43 min , y  = $24.41
     24.41 =a*43+b...........eq(1)
 when t= 82 min , y=  $19.34
19.34 = a*82+b....................eq(2)
*[invoke linear_substitution "a", "b", 43, 1, 24.41, 82, 1, 19.34 ]
 a=-0.13 , b =30
  hence y= (-0.13)t+30
    equation becomes  y =-0.13t+30
 when t = 103  min, y= -0.13*103 +30
                = $16.61
 Result:Remaining credit after 103  min  = $ 16.61