Chicken and rabbit are in the same cage, with 45 heads on the top and 126 feet on the bottom?
Suppose all rabbits, then chickens have: (45 × 4-126) △ 4-2, = 54 △ 2, = 27 (rabbits), then rabbits have: 45-27 = 18 (rabbits), answer: there are 27 chickens and 18 rabbits