Question 818906
The given information only allows to find the price versus box size.  You can also find the price per ounce of bran flakes.

(size, price) as points using variables, (x,p) where p is price and x is size.


The two data points are (10,3) and (15,4).  Fit to a line equation.
slope is {{{(4-3)/(15-10)=1/5}}}
{{{p=(1/5)x+b}}}
{{{b=p-(1/5)x}}}
Pick any point to find b.
{{{b=3-(1/5)10}}}
{{{b=3-2}}}
{{{b=1}}}
EQUATION: {{{highlight(p=(1/5)x+1)}}}


A box size for 0 ounces costs 1 dollar.  According to the slope, each ounce of blan flakes is (1/5) of a dollar or 0.20 dollars.  This function p(x) still indicates 1 dollar to be added to this.  If a box were made to hold 5 ounces, then the price for this box would be {{{p=(1/5)5+1=highlight(2)}}} dollars.