Lifespan

The lifetime of a light bulb is a random variable with a normal distribution of x = 300 hours, σ = 35 hours.
a) What is the probability that a randomly selected light bulb will have a lifespan of more than 320 hours?
b) To what value of L hours can the lamp life be expected to last longer than L hours with a probability of 0.25?

Correct answer:

p =  0.2839
L =  323.6 h

Step-by-step explanation:

$$\begin{gathered} \mu =300\text{h} \\ \sigma =35\text{h} \\ x\dots N(\mu ,\sigma ) \\ t>320h \\ p=0.2839 \end{gathered}$$

$L=323.6=323.6\text{h}$

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