Question 1152386
given:

The monthly cost (in dollars) of water use is a linear function:

{{{y=mx+b}}}  where  {{{x}}} represents {{{HCF}}} and {{{y}}} represents the {{{cost}}} for {{{using}}}

if the cost for using {{{x=16}}} HCF of water is ${{{y=34.60}}}, means
that is a point ({{{16}}},{{{34.60}}})

the cost for using {{{x=30}}} HCF is ${{{y=57.70}}}, means
that is a point ({{{30}}},{{{57.70}}})

use points to find slope {{{m}}}

{{{m=(57.70-34.60)/(30-16)=23.1/14=1.65}}}

=>{{{y=1.65x+b}}} 

to find {{{b}}}, use one point above

{{{34.60=1.65*16x+b}}}

{{{34.60=26.4x+b}}}

{{{34.60-26.4x=b}}}

{{{b=8.20}}}

and, your equation is: {{{y=1.65x+8.20}}} 

use equation to calculate what is the cost for using {{{x=28}}} HCF of water is:

{{{y=1.65*28+8.20}}} 

{{{y=46.20+8.20}}} 

{{{y=54.40}}} 

the cost for using {{{28}}} HCF of water is {{{54.40}}} 

{{{drawing ( 600, 600, -10, 70, -10, 70,
circle(16,34.6,.3),locate(16,34.6,p(16,34.60)),
circle(30,57.7,.3),locate(30,57.7,p(30,57.7)),
circle(28,54.4,.3),locate(28,54.4,p(28,54.4)),
graph( 600, 600, -10, 70, -10, 70, 1.65x+8.20)) }}}