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 42 minutes of calls is $14.12 , and the remaining credit after 74 minutes of calls is $9.64 . What is the remaining credit after 78 minutes of calls?

Question

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  42 minutes of calls is $14.12  , and the remaining credit after  74 minutes of calls is $9.64  . What is the remaining credit after 78 minutes of calls?

anonymous · Answer

ii got to $14.92-9.64=5.28 and 74-42=32 but now im lost

anonymous · Answer

it may be
  9.64 -  ( ($14.12 -9.64)/(74-42)   *(78-74)  )

anonymous · Answer

9.64-0.56=9.08

anonymous · Answer

what about this one @matricked 
Milan is driving to Boston. Suppose that the distance to his destination (in miles) is a linear function of his total driving time (in minutes). Milan has 68 miles to his destination after 19 minutes of driving, and he has 53.6 miles to his destination after 35  minutes of driving. How many miles will he have to his destination after 49  minutes of driving?

anonymous · Answer

it will be  53.6  -  (((68-53.6) / (35-19)) * (49-35) )=53.6-12.6=41