How to count number of animals with 4 legs and 2 legs ?

Hi, how should I calculate the number of animal heads, if I know the total number of animals and total number of legs ? There are 2 groups of animals. Some of them have 4 legs and others have 2 legs. In total there are 36 animals and they have 100 legs. How do I know how many animals have 4 legs and how many animals have 2 legs ? What formula should I use ?

To calculate how many animals have 4 legs or 2 legs, if you know the total number of animals and total number of their legs, you have to use 2 equations.

Calculation

Total number of animals: 36
Total number of legs: 100

x - represents animals with 4 legs
y - represents animals with 2 legs

Equation 1
Total number of animals: x + y = 36

Equation 2
Total number of legs: 4x + 2y = 100
Result of this equation (divided by 2): 2x + y = 50

Equation 1 and Equation 2:
(2x + y) - (x + y) = 50 - 36
Result of this equation: x = 14

There are 14 animals with 4 legs.

Other 22 animals have 2 legs.