Since Kirk starting reading on Saturday, I can make an equation of 20 + 20d. The equation for Lito is 20d. Therefore, the system of equations is 20 + 20d = 25d.
I can solve this like any other equation. Firstly, I can subtract 20d on the left hand side to give me 20 = 5d. Then, I can divide by 5. This gives me 4 = d, or d = 4. This means that it takes Lito 4 days to catch up. I can then multiply 20 * 4 and 25 * 4 to give me 100 pages.
The system of equations is 20 + 20d = 25d, and the amount of pages they each read is 100.