Mean of 100 items is 49 . It was discovered that three items which should have been 60,70,80 were wrongly read as 40 , 20,50 respectively. The correct mean is

Mean of 100 items =x1+x2+.....x97+40+20+50100=49                                      ⇒   x1+x2+.........+x97=4900-110=4790                         correct mean =x1+x2........+x97+60+70+80100 =4790+210100=5000100=50