Question 697428
Basically, they are giving you 2 points on a graph, and
they want you to find a 3rd point.
Plot hundreds of ft3 ( HFC ) on the horizontal axis
and Monthly cost on the vertical axis.
---------------
Given:
( 17, 34.47 ) and
( 45, 80.67 )
------------
Let {{{ c }}} = Monthly cost of water use
Let {{{ h }}} = HFC's of water
------------
Use the general point-slope formula
{{{ ( c - 34.47 ) / ( h - 17 ) = ( 80.67 - 34.47 ) / ( 45 - 17 ) }}}
{{{ ( c - 34.47 ) / ( h - 17 ) = 46.2 / 28 }}}
{{{ 28*( c - 34.47 ) = 46.2*( h - 17 ) }}}
{{{ 28c - 965.16 = 46.2h - 785.4 }}}
{{{ 28c = 46.2h + 179.76 }}}
{{{ c = 1.65h + 6.42 }}} answer
-------------------
check answer:
Does the line include ( 45, 80.67 )
{{{ c = 1.65h + 6.42 }}}
{{{ 80.67 = 1.65*45 + 6.42 }}}
{{{ 80.67 = 74.25 + 6.42 }}}
{{{ 80.67 = 80.67 }}}
OK- you can check ( 17, 34.47 )
----------------------------
What is the cost for using 25 HCF of water? 
{{{ c = 1.65*25 + 6.42 }}} 
{{{ c = 41.25 + 6.42 }}}
{{{ c = 47.67 }}}
The cost is $47.67