Bulb life

Tests show that the lives of light bulbs are normally distributed with a mean of 750 hours and a standard deviation of 75 hours. Find the probability that a randomly selected light bulb will last between 675 and 900 hours.

Final Answer:

p =  0.818

Step-by-step explanation:

$$\begin{gathered} m=750\text{h} \\ \sigma =75\text{h} \\ p=N(675<X<900,m,\sigma ) \\ {m}_2=m-2\cdot \sigma =750-2\cdot 75=600 \\ {m}_4=m+2\cdot \sigma =750+2\cdot 75=900 \\ 1\sigma -2\sigma -3\sigma rule \\ p=2\cdot 0.341+0.136=0.818 \end{gathered}$$

Use the normal distribution table.

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