                                                                  
                                                                                       
                                                                                                       
                                                                                         
                                                            6  11                            
                                                        
                                            750   \         50   \              8—1             750               750＋50＝800   \                           750－50＝700   \     
               5  6                                                                     c  3×108  \                                         c                                      