There are 100 chickens and rabbits. There are 70 less feet in chicken than in rabbit. How many chickens and rabbits are there? There should be a formula

If the chicken has x feet, the rabbit has x + 70 feet
Because the chicken has two feet and the rabbit has four
The number of chickens is x / 2 and the number of rabbits is (x + 70) / 4
X/2+(X+70)/4=100
The solution is x = 110
There were 55 chickens in 110 / 2
The number of rabbits was 100-55 = 45

There are 100 chickens and rabbits. The number of feet of rabbits is 40 more than that of chickens. How many chickens and rabbits are there?

Suppose there are x chickens
=>There are 100-x rabbits
Rabbits have 40 more feet than chickens
=>(100-X)*4=2X+40
=>X = 60
Chicken 60, rabbit 40

There are 27 chickens and rabbits. The number of feet of rabbits is 18 more than that of chickens. How many chickens and rabbits are there?
We need ideas, formulas, not equations

1. Arithmetic
Rabbit: (27 * 2 + 18) / (2 + 4) = 12
Chicken: 27-12 = 15
Solving the second equation
Suppose there are x and Y chickens and rabbits respectively
Then x + y = 27,4y-2x = 18
The solution is x = 15, y = 12
There are 15 chickens and 12 rabbits

There are 100 chickens and rabbits. There are 70 less feet in chicken than in rabbit. How many chickens and rabbits are there? There should be a formula