q55 is A. Product means multiplied and sum means added.
As 0.75 is a common factor you can put it into brackets to make 0.75 * (r + $3.20)
q50 was a question answered about half an hour ago strangely.
so $15/$150 simplifies to $1/$10 so he makes (total commission, C) 1/10 of the shoes' cost (s)
Hope this helps, comment with questions :)
