The length of human pregnancies from conception to birth can be approximated a normal distribution with a mean of 265 days and a standard deviation of 15 days. What percentage of all pregnancies will last between 250 and 280 days?

Normal/Gaussian curve: Is ‘bilaterally symmetrical, bell-shaped’ Is bases on mean (µ) and standard deviation (σ)  Mean, Median and Mode coincide (Mean = Median = Mode) Has Mean (µ) = 0 SD (σ) = 1 Mean ± 1SD (µ ± 1σ) covers 68% values Mean ± 2SD (µ ± 2σ) covers 95% values Mean ± 3SD (µ ± 3σ) covers 99% values - In a Normal distribution, 50% of values lie above the mean and 50% lie below the mean. Mean is 265 days , SD = 15 Mean + 1 SD = 250, 280 As Mean + 1 SD covers 68% values. So percentage of all pregnancies will last between 250 and 280 days is 68%.