<prompt> The PEFR of a group of 11 year old girls follow a normal distribution with mean 300 lmin and standard detion 20 lmin :About 95 of the girls have PEFR between 260 and 340 lminThe girls have healthy lungsAbout 5 of girls have PEFR below 260 lminAll the PEFR must be less than 340 1min  <response>: S.D AREA BETWEEN S.D One S.D 68.3 Two S.D 95.4 Three S.D 99.7 So, 95, about girls have PEFR between two S.D Given in question mean 300Lmin S.D 20Lmin A. Girls between one S.D 68.3 300- 1 x20 320 to 280 So, 68.3 girls have PEFR between 320 and 280Lmin B.Girls between two S.D 95.5 300- 2 x 20 340 to 260 So, 95 girls have PEFR between 260 and 340Lmin C. Girls between three S.D 99.7 300 - 3 X 20 300- 60 360 to 240 Ref: Parks Textbook Of Preventive And Social Medicine By K. Park, 19th Edition, Page 402 