The book has a total number of 156 pages. If he reads 12 pages per night, we need to multiple the total number of days by the amount he reads per night in order to get 156. Since we don't know how many days it will take, we will use d to represent the amount of days. So, the equation ends up looking like this:
12d = 156
d represents the amount of days it will take for Carlos to read the book.
