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### Ben Evans says

On a normal a normal distribution curve 34.18% lie within one standard deviation above the mean and bellow the mean. 13.68% lie between one and two standard deviations. 2.1% lie between the two and three standard deviaitons.

If the standard deviation is 13.5 cm, then 561 would be 2 standard deviations. 1000 (50%-34.18%-13.68%)=21.4

Between 520.5 is three standard deviation less than the mean and 547.5 is one standard deviation from the mean. The number of observations would be 1000(13.68%+2.1%)=157.8