Answer:
 1) The graph labeled and the ordered pairs plotted are shown in the second picture.
 2) To calculate the amount of meat (in pounds) needed for a party of 75 people: 
 Substitute the value of the Constant of proportionality and 
 into the equation 
 and then evaluate (The result is 30 pounds).
Explanation:
 The missing graph is attached.
 Proportional relationships have the following form:
 

 Where "k" is the Constant of proportionality.
 1) You can observe in the graph shown in the first picture this point:
 

 So you can subsitute the coordinates of this point into 
 and solve for "k":
 

 Now you can find the amount of meat needed for 20, 30, 40, and 50 people with this procedure:
 For 20 people
 Substituting 
 and 
 into the equation 
 and evaluating, you get:
 

 The ordered pair is:
 

 For 30 people
 Substituting 
 and 
 into the equation 
 and evaluating, you get:
 

 The ordered pair is:
 

 For 40 people
 Substituting 
 and 
 into the equation 
 and evaluating, you get:
 

 The ordered pair is:
 

 For 50 people
 Substituting 
 and 
 into the equation 
 and evaluating, you get:
 

 The ordered pair is:
 

 Knowing those values of "y"and having the ordered pairs, you can finish labeling the graph an plotting the ordered pairs (See the second picture.).
 
 2) Substituting 
 and 
 into the equation 
 and evaluating, you can calculate the amount of meat (in pounds) needed for a party of 75 people. This is:
 
