15.87 % percent of batteries have lifetimes more than 561 days.
Given:
battery lifetime is normally distributed for large samples. the mean lifetime is 500 days and the standard deviation is 61 days.
μ = 500
σ = 61
we know that
z = X - μ / σ
z score :
A Z-score is a numerical measurement that describes a value's relationship to the mean of a group of values.
Given X = 561 days
substitute
z = 561 - 500 / 61
= 61 / 61
z = 1
At z = 1 p value is 0.8413
1 - 0.8413 = 0.1587
= 0.1587 * 100 %
= 15.87 %
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