Answer:
C. She wants to show by what percent the book was discount.
Step-by-step explanation:
We are given that,
The expression showing the percent of the original cost = [tex]0.7b[/tex]
Re-writing the expression, we have,
The expression showing the percent of the original cost = [tex](1-0.3)b[/tex] = [tex]b-0.3b[/tex]
So, we get the total price 'b' is reduced by 30%.
Thus, the expression shows the discount she received on the book is 30%.
Hence, Jessica re-writes the expression to show the percent by which the book was discounted.