1,500 pages in 12 days = _____ pages in 3 days.

You need to find out how many pages you can read in one day. Divide the total pages by the number of days.... then you can find out how many pages you can read in 3 days.

Two ways at least to do it.

first
1500/12 = ? / 3
divide top and bottom of the fraction on the left by 4
376/3 = ?/3
well ? seems to be 375

second
how many pages per day?
1500/12 = 125 pages per day
125 * 3 days = 375