Given:
Animals: Chicken and Rabbits
Total heads : 72
Total feet : 200
Chicken and Rabbits have one head : C + R = 72
Chicken have 2 feet and Rabbits have 4 feet : 2C + 4R = 200
C + R = 72
C = 72 - R
2C + 4R = 200
2(72 - R) + 4R = 200
144 - 2R + 4R = 200
-2R + 4R = 200 - 144
2R = 56
R = 56/2
R = 28
C = 72 - R
C = 72 - 28
C = 44
2C + 4R = 200
2(44) + 4(28) = 200
88 + 112 = 200
200 = 200