Answer: all the ones that do not have a negative sign in front easy.
((5x2)+5.5+(6x3)+6.5+(7x5)+(7.5x4)+(8x4))/20
137/20
= 6.85
The answer is 35.
........
Discount prices/the discount
E.g.the new price of the book is 75 which has a discount of 75%
The original price = 75/0.75=100