let

c = the number of chickens on the farm

p = the number of pigs on the farm

each chicken has 2 feet there are total of 2c chicken feet on the farm.

each pig has 4 feet there are total of 4p pig feet on the farm.

altogether there are 200 feet on the farm:

2c + 4p = 200 divide both sides by 2

c + 2p = 100

each chicken and pig has 1 head thus the total number of heads is: c + p and

c + p = 72

by solving the system of equations:

c + 2p = 100

c + p = 72

we find:

c = 44 chickens

p = 28 pigs

click here to see the step by step solution of the system of equations:

there are 44 chickens and 28 pigs on Stan's farm.