We are given the number of questions Sarah wrote per day. Let's make the fourth-day x
First day =12 pages
Second day = 15 pages
Third day =9 pages
Fourth day = x pages
Since she wrote 12 pages per day, this implies the average of all the pages she wrote over the course of four days.
Here is the formula:
[tex]\text{pages per day =}\frac{sum\text{ of pages}}{\text{total number of days}}[/tex]Therefore,
[tex]\begin{gathered} 12=\frac{12+15+9+x}{4} \\ 12=\frac{36+x}{4} \\ 12\times4=36+x \\ 36+x=48 \\ x=48-36 \\ x=12 \end{gathered}[/tex]ANSWER: OPTION A(12 Pages)