Final answer:
To find the number of cats and dogs boarded, you can use the equations x + y = 18 and 20x + 25y = 400.
Step-by-step explanation:
To find the number of cats, x, and dogs, y, that were boarded, we can set up a system of equations based on the given information.
Let's use x to represent the number of cats and y to represent the number of dogs.
The first equation should reflect the total number of animals boarded: x + y = 18.
The second equation should reflect the total amount earned: 20x + 25y = 400.
Therefore, the correct equations to find the number of cats and dogs boarded are x + y = 18 and 20x + 25y = 400.