Goi so doan xe luc dau x（xe）（x>0)
Du dinh xe tro duoc 30/x (tan)
Thuc te nhan them 2 xe nua :(x+2)(xe)
thuc te xe tro duoc 30/x+2 (tan)
theo bai ra ta co phuong trinh :
30/x -30/x+2 = 1/2
<=> 1/2x^2 -x +60 =0
<=> x1 = 10 t/m)
       x2 =-12 (loai )
vay so xe luc fau la 10 xe