Question 1174250
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A medical supply company found out that there is a demand on a new brand of alcohol in the market.
There was demand for 500mL and 1L with the amount Php 80 and Php130, respectively
  (A) What is the slope?
  (B) What is the linear function represent the given problem if x is the number of alcohol 
      and y is the price of alcohol?
  (C) How much does 750mL cost?
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The solution by @CPhill is NOT CONSISTENT with the problem's requirements.


Definition of the notion/conception of "demand" in Economics:


<pre>
    In economics, demand is the quantity of a good or service 
    that consumers are willing and able to buy at a given price.
</pre>


Simply saying, the amount of goods is in the numerator,
while the money (price) is in the denominator.



<pre>
The given points are  (Php80, 500 mL)  and  (Php130, 1000 mL).


(A)  The slope is  m = {{{(1000-500)/(130-80)}}} = {{{500/50}}} = 10 mL of the alcohol for 1 Php.    


                   It is the  <U>Answer to question A</U>.



(B)  The function representing the given problem if x is the amount of alcohol 
     and y is the price of alcohol  {{{highlight(highlight(IS))}}}

         x = a + my = a + 10y.    (1)


     Here "a" is the unknown coefficient.  To find it, substitute the numbers into the first equation

         500 = a + 10*80      (2)

     and find 

         a = 500 - 10*80 = 500 - 800 = -300.


    So, the demand equation is 

          x = -300 + 10y.     (4) 


                   It is the  <U>Answer to question B</U>.



(C)  How much does 750mL cost?

           750 = -300 + 10*y  --->  750 + 300 = 10y  --->  1050 = 10y  --->  y = 1050/10 = 105.


                   <U>ANSWER to question (C)</U>:  750 mL of alcohol cost  Php 105,

                                                                 which is precisely half-way between Php 80 and Php 130,
                                                                 exactly as it should be, based on common sense.
</pre>

Solved.



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This problem is an OBVIOUS TRAP.


The trap is made simply by permutation of letters x and y in the problem's formulation.