Question 1113193
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For a three pound package it costs $6.50 and for a 7 pound package it costs $12.50.
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x, size in weight
y, cost of package


Points (x,y):
(3, 6.5)  and  (7, 12.5).


{{{m=(12.5-6.5)/(7-3)}}}
{{{m=6/4=3/2}}}


{{{y=mx+b}}}
{{{y-mx=b}}}
{{{b=y-mx}}}


{{{b=6.5-(3/2)*3}}}
{{{b=6.5-9/2}}}
{{{b=6.5-4.5}}}
{{{b=2}}}


{{{highlight(y=(3/2)x+2)}}}

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A rate per pound of $1.5 and a fixed fee of $2 for any size package