x/20+y/30=5 两边乘60
3x+2y=300
2y=300-3x
y=150-1.5x
y/20+x/30=7 两边乘60
3y+2x=420
3(150-1.5x)+2x=420
450-4.5x+2x=420
2.5x=30
x=12
y=150-1.5x12=150-18=132