1 doi cong nhan co 120 nguoi dap 1doan duong dai 4km . Ban chi huy tinh rang de hoan thanh cong viec dung ke hoach thi moi ngay moi nguoi phai lam 8 gio . Truoc khi khoi cong , thi them 30 nguoi ve phai dap them 1 km duong nua . Hoi de hoan thanh thi moi nguoi phai lam bao nhieu gio ?