鸡有2条腿，兔子有4条腿 假设鸡的数量为c只，兔子的数量为r只

根据以上条件确定 c+r = 30 2c + 4r = 90

2c+4r有一个公因数2

2(c+2r) = 90

c+2r = 90/2 = 45

2r是2*r,本质上r+r

所以c+2r = c+r+r = 30 + r = 45

r = 15 c+r = c+15 = 30 c = 15

根据上述计算，将x,y带入公式

c+r = x 2c+4r = y

2(c+2r) = y

c+r+r = y/2

x + r = y/2

r = y/2 - x c = x - r