n has less than 75 copies
sold 12
so add the 12 back 
n had less than 87 copies in the beginning of the day
:)
 
        
                    
             
        
        
        
Answer:
By using Carl Gauss's clever formula, (n / 2)(first number + last number) = sum, where n is the number of integers, we learned how to add consecutive numbers quickly. We now know that the sum of the pairs in consecutive numbers starting with the first and last numbers is equal.
 
        
             
        
        
        
Answer:
378
Step-by-step explanation:
9*6=54 then 54/2=27. fnally 27*14=378
 
        
                    
             
        
        
        
I personally use y2-y1 over x2-x1 because it's simple.  Plot two y values and two x values.  The first ones you plotted are x1 and y1, the others are x2 y2.  Now subtract y2-y1 and x2-x1.  Afterwards, divide the y value by the x value, that will give you slope.