For this case we can propose a system of equations:
x: Let the variable that represents the number of small cookies
y: Let the variable that represents the number of large cookies
According to the statement we have:

We multiply the first equation by -2:

We have the following equivalent system:

We add the equations:

We look for the value of the variable "x":

Answer:
They sold 80 small cookies and 50 large cookies
Well it would actually be 7.2 inches per each piece
After moving the 1 1/2 to the other side you will be able to solve it and get x=2