A laptop company claims up to 9.1 hours of wireless web usage for its newest laptop battery life. However, reviews on this laptop show many complaints about low battery life. A survey on battery life reported by customers shows that it follows a normal distribution with a mean of 8.5 hours and a standard deviation of 39 minutes.

Answer :

Given information :

The population mean $(μ) = 8.5 

The population standard deviation $(σ) = 3960 = 0.65 

Solution :

The probability that the battery life is at least 9.1 hours :

$$\text{P}(x \geq 9.1) = \text{P}\left(\frac{x – \mu}{\sigma} \geq \frac{9.1 – 8.5}{0.65}\right)$$ $$= \text{P}(z \geq 0.923)$$ $$= 1 – \text{P}(z < 0.923)$$ $$= 1 – 0.822$$ $$ = 0.178$$