6000/(x+50)+3000/x=9000/(x+30)
化简：2/(x+50)+1/x=3/(x+30)
等式两边同乘x(X+50)(X+30)
2X(X+30)+(X+50)(X+30)=3x(X+50)
2X^2+60x+x^2+80x+1500=3x^2+150x
x=150