The lifetimes of projector bulbs of a particular type are normally distributed with a mean of 470 hours and a standard deviation of 15 hours. What percentage of the bulbs has lifetimes that lie within 2 standard deviations of the mean on either side?

Question

Apply the 68-95-99.7 rule to this question.

Answer 1

The "68-95-99.7 rule" represents the two-tail probability of data within ±1,2 and 3 standard deviations respectively.
For example, the probability of picking a value X from a normal distribution within the range μ-σ<Xμ+σ is approximately 68%.

Using this rule, you will not need the normal distribution table to provide an answer to the question.

Answer 2

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