need help

Hey!

Let's start by setting two variables, 

$S = \text{sheep}$

$C = \text{chicken}$

Once we have our variables we can go ahead and make our equations:

1.) The easiest equation that we can make with these two variables is with the total # of animals.

The number of sheep + The number of chickens = The total # of animals,

So we get the equation: $S + C = 20$.

2.) Now, we are given that sheep have $4$ legs and chickens have $2$ legs.

(The number of sheep x The number of legs each sheep has) + (The number of chickens x The number of legs each chicken has) = The total # of legs.

So we get the equation $4S + 2C = 64$

Now that we have our two equations,

$4S + 2C = 64$

$S + C = 20$,

You can use the two equations to either substitute the variables or eliminate them to find your answer.