The PEFR of a group of 11 year old girls follow a normal distrubution with mean 300 1/min and standard deviation 20 1/min -
About 95% of the girls have PEFR between 260 and 340 1/min. The girls have healthy lungs About 5% of girls have PEFR below 260 1/min. All the PEFR must be less than 340 1/min.