桃3,李20,橄榄77
设买桃用的钱为x,买李的钱为y,买橄榄的钱为z,列方程组:
(1):
x+y+z=100;
(2):
x/3+y/4+7z=100;
(2)*3:
x+3y/4+21z=300;
(1)-(2)*3:
y/4-20z+200=0;
消掉x,得y=80(z-10).
因为0