(20 )A clsss of pupie takes a speling tesL Thelr mean score for the test is mr Anoiher PuPH TekeS the test and scores n. When this pupil's result is included with the other results it is found tht the mean has decreased by 2. Which equation below gives the correct expression for r in terms of 7 7 A 2二 B ーー 如一3 2 世一3 7ー2 ァニーーー 3 芝填2 カルニーーー 直一1 如 ォ=ニー で し 加十2 、れ=ーテ=ー