So, you already know that angle CBD is 68 degrees, and the complementary angle to angle CBD is angle CBA. These angles together make 180 degrees because segment DBA is a straight line consisting of angles CBD and CBA. 180-68(the angle you already know) is 112 degrees for angle CBA. Then you repeat this process to find the other angles
Let N be the number of items sold and p the price.
Since the variation is inverse, then the relation between N and p is:

For N=20000 and p = $9.5, we get the formula:

If p = 8.75, then the number of items sold can be computed using the formula: