2x+4(20-x)=70

Solution:

x=5，20-x= 15 .

Substitute into the original formula and verify that it meets the meaning of the question.

So there are five chickens, 15 rabbits.

(2) Leg lifting method

This is the method used by the ancients to solve problems, which is also the method used in Sun Tzu's calculation.

1, leg lift, that is, the chicken is "golden rooster independent", the rabbit has two hind legs touching the ground and its front legs raised, so the number of legs is half of the original.

Now the chicken has one foot and the rabbit has two feet. As long as there is a rabbit in the cage, its feet are more than its head 1.

Then the difference between the number of feet and the number of heads is the number of rabbits.

4. Finally, the number of chickens is obtained by subtracting the number of rabbits from the number of human heads.

So we can sum up the formula: the number of rabbits = the total number of legs ÷2- the total number.

That is, the number of rabbits = 70 ÷ 2-20 = 15

Number of chickens =20- 15=5

(3) Hypothetical method

Suppose all rabbits have 20*4=80.

It's over 70, so where is it? Of course, we regard a two-legged chicken as a four-legged rabbit. As we all know, rabbits have two more legs than chickens, and two more legs are 1 chicken, so there are as many chickens as there are two legs.

Number of chickens (80-70)÷2=5

Number of rabbits 20-5= 15

(4) Leg cutting method

Leg amputation method is an in-depth expansion of hypothesis method. Let's cut off the legs of each chicken and rabbit first, so that each chicken has no legs, and each rabbit has two legs left, and the total number of legs becomes 70-20× 2 = 30 (strips), then these 30 legs are the legs of rabbits with two legs cut off.

So the number of rabbits is 30÷2= 15 (only).

The number of chickens is 20- 15 = 5 (only).