Solution to An electrical firm manufactures light bulbs that have a length of life that is approximately … - Sikademy
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Archangel Macsika

An electrical firm manufactures light bulbs that have a length of life that is approximately normally distributed, with mean equal to 808 hours and a standard deviation of 93 hours. Find the probability that a random samples of 19 bulbs will have an average life of greater than 799 hours.

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Let X= an average life: X\sim N(\mu, \sigma^2/n).

Given \mu=808\ h, \sigma=93\ h, n=19.


P(X>799)=1-P(X\le 799)

=1-P(Z\le \dfrac{799-808}{93/\sqrt{19}})

\approx 1-P(Z\le-0.421829)

\approx0.663425

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