Expert: Lynn Houston - 9/5/2008

Question
At noon one day, the river peaked at 11 feet above flood stage. It than began to recede its depth dropping at 4 inches per hour.
(a) at 3:30 that afternoon how many inches above the flood stage was the river
(b) let "t" stand for the number of hours since noon, and "h" stand for the corresponding number of inches that the river was above flood stage.  Make a table of values and write an equation that expresses "h" in terms of "t".
(c) Plot "h" versus "t" with "t" on the horizontal axis
(d) for how many hours past noon was the river at least 36 inches above flood stage.

Answer
a)  noon to 3:30 is 3.5 hrs, 3.5*4=14 inches, so the river is still 9'10" above flood stage.

b) t     h

 1      4
 2      8
 3      12

h=4t

c) you're going to have to do on your own (can't really do graphs here), but all you need to do is draw two perpendicular lines and use the values in the table to plot points.  T is going to be the horizontal line.

d) 36" = 3ft, so the question is, how long did it take the river, at 4" per hr, to recede 8 feet (or 96 inches).  96/4 = 24 hrs.

Let me know if something isn't clear.