Answer:
C. She wants to show by what percent the book was discount.
Explanation:
We are given that,
The expression showing the percent of the original cost =
![0.7b](https://img.qammunity.org/2020/formulas/mathematics/high-school/ewve3qt8cw87lumcufur5b1g16pmf0mi7r.png)
Re-writing the expression, we have,
The expression showing the percent of the original cost =
=
![b-0.3b](https://img.qammunity.org/2020/formulas/mathematics/high-school/vfxbiuoqo3kxpxspe32zuv119fsz0t72ja.png)
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.