Equation about number of animals Equation about number of legs Solve the system

Given information:
Total animals  = 13
Total legs = 40

Assumed all animals have their legs intact ... 
Pig = 4 legs
Chicken = 2legs

Assumed all chickens:
Total legs = 2 x 13 = 26
Extra legs = 40 - 26 = 14
Extra legs belongs to the pig (each pig has extra 2 legs)
14 ÷ 2 = 7 
There are 7 pigs
13 - 7 = 6 
There are 6 chickens

---------- OR ALGEBRAICALLY --------

Let the number of pigs be x and number of chickens be y
pigs = x
chickens = y
x + y = 13 -------------- (1)
4x + 2y = 40 ---------- (2)
From (1):
x + y = 13
x = 13 - y -------------- sub into (2)
4(13 - y) + 2y = 40
52 - 4y + 2y = 40
52 - 2y = 40
2y = 52 - 40
2y = 12
y = 6 ----------- sub into (1)
x + 6 = 13
x = 13 - 6
x = 7

x = 7, y = 6
pigs = 7, chickens = 6