Do the instructions tell you to model this scenario using an equation or to write a formula?
I don't know what your class is doing right now, but simply dividing the number of remaining pages by the number of pages read-per-day will give you the answer.
Division "undoes" multiplication.
That is, if I told you that I read 50 posts per day for 30 days, then the total number of posts that I read is 50×30=1500.
Now think in reverse. If I were to have told you instead that I need to read 1500 posts AND that I will read 50 posts-per-day, how many days would it take me to read them all?
1500/50 = 30
Likewise, you need to divide the remaining pages to be read by the number of pages read per day.