You know that the farmer has 18 total animals, so x + y = 18.
You know that cows have 4 legs and chickens have 2 legs, and the 18 cows and chickens have 60 legs total, so 4x + 2y = 60.
Now, to “solve” the system of equations, you can take the simpler equation x+y = 18, and simplify it, to stand in for variables.
Since we want to find y, the number of chickens, let’s make this equation = x.
So, it is now x = 18 – y.
Now you can take 18-x and put it into the OTHER equation to stand in for y, so that you can solve for y.
4(18-y) + 2y = 60
Distribute around the parentheses…
72 – 4y + 2y = 60
Combine the y’s.
72 – 2y = 60
-72 from both sides
-2y = -12
Divide by -2!
y = 6
So, there are 6 chickens.
And, if you wanted to find the number of cows, you can plug the value for y into the equation again… 6 + x = 18 and solve for x.