There are 13 animals in a barn. Some are Chickens and some are Pigs. Altogether
they have 40 legs. How many of each animal are in the barn?
There are 5 pigs and 8 chickens.
There are 8 pigs and 5 chickens.
There are 7 chickens and 6 pigs.
There are 7 pigs and 6 chickens.

Ahh! What a classic: Systems of Equations!

So, if you don't know what a system of equations is, its basically two equations with two variables combined into a system.

So, let x=# of pigs

And also, Let y=# of chickens

Let's make our first equation!

This is because there are 13 animals in a barn.

So then, knowing that a pig has 4 legs and a chicken 2:

This is because there are 40 legs in total, and we multiply the variable because that's how many legs each creature has.

So, here is our system!

Through Substitution (the inputting of something instead of a variable):

Then, we input it into the next equation:

This means that there are 6 chickens, and by inputting it into the first equation:

So there you have it!

FYI there are 6 chickens and 7 pigs.

Hope this helps!

P.S. Stay Safe!