Chickens are 20 more than rabbits, with 20*2=40 feet.

There are as many chickens as rabbits. One has feet: 280-40=240.

The remaining chickens and rabbits are 240/(4+2)=40.

As it turns out,

Chicken: 40+20=60.

There are rabbits: 40

2.

There are 20 more chickens than rabbits. If there were no rabbits,

Chicken feet are 20*2=40 more than rabbit feet.

Every time you add a rabbit, you should also add a chicken (the difference between chicken and rabbit is 20, and this will not change)

Four rabbit feet and two chicken feet were added.

Their difference is reduced by 4-2=2.

From 40 chicken feet more than rabbit feet to 40 chicken feet less than rabbit feet,

One * * * change: (40+40)/2=40 times.

So there are 40 rabbits.

There are 40+20=60 chickens.