Please tell me a chicken and rabbit joke

It is known that chickens and rabbits have 15 legs and 40 legs. How many chickens and rabbits are there? Algorithm: Assume that the chickens and rabbits are well-trained, blow a whistle, they lift one foot, 40-15=25:; blow the whistle again, they lift the other foot, 25-15=10, at this time The chicken fell to the ground, but the rabbit still had two legs. So the rabbit is 10÷2=5; the chicken is 15-5=10. This kind of algorithm is a linear equation of two variables. . . . . .