设井深x尺,绳长y尺
y/3 - x =4
y/4 - x =1
由第二个方程,x = y/4 - 1
代入第一个方程,y/3 - ( y/4 - 1) =4
y/12= 3
y=36
x= y/4 -1=8
井深8尺,绳长36尺