<prompt> The PEFR of a group of 11 year old girls follow a normal distribution with mean 300 1min and standard deviation 201minAbout 5 of girls have PEFR below 260 lminThe girls have healthy lungsAbout 95 of the girls have PEFR between 260 and 340 lminAll the PEFR must be less than 340 lmin  <response>: In Normal distribution, Mean 1SD m 1σ covers 68 values Mean 2SD m 2σ covers 95 values Mean 3SD m 3σ covers 99 valuesIn the given question,Mean of PEFR μ 300 lminStandard deviation of PEFR σ 20 lminThus,68 of girls will have PEFR in the range Mean 1SD m 1σ 300 20 lmin, i.e. between 300 20 and 300 20 1min 280 320 lmin range covers 68 of girls95 of girls will have PEFR in range Mean 2SD m 2σ 300 220 lmin, i.e. between 300 40 and 300 40 1min 260 340 lmin range covers 95 of girls99 of girls will have PEFR in range Mean 3SD m 3σ 300 320 lmin, i.e. between 300 60 and 300 60 1min 240 360 lmin range covers 99 of girlsNow, 260 340 lmin range covers 95 of girls, thus rest 5 of girls are outside this range, i.e. they have PEFR either 260 lmin or 340 lmin Even this will be symmetrically distributed Normal curve is bilaterally symmetrical, thus2.5 girls will have PEFR 260 lmin2.5 girls will have PEFR 340 lmin Girls having PEFR less than 340 lmin: 97.5Since the normal range of PEFR for girls is not given in the question, it cannot be concluded that all girls have healthy lungs. 