A quality control department checks the lifetimes of batteries produced by a company.
The lifetimes, \(x\) minutes, for a random sample of 80 'Superstrength' batteries are shown in the table below.
| Lifetime | \(160 \leq x < 165\) | \(165 \leq x < 168\) | \(168 \leq x < 170\) | \(170 \leq x < 172\) | \(172 \leq x < 175\) | \(175 \leq x < 180\) |
| Frequency | 5 | 14 | 20 | 21 | 16 | 4 |
- Estimate the proportion of these batteries which have a lifetime of at least 174.0 minutes. [2]
- Use the data in the table to estimate
[3]
The data in the table on the previous page are represented in the following histogram, Fig 15.
\includegraphics{figure_15}
A quality control manager models the data by a Normal distribution with the mean and standard deviation you calculated in part (b).
- Comment briefly on whether the histogram supports this choice of model. [2]
- Use this model to estimate the probability that a randomly selected battery will have a lifetime of more than 174.0 minutes.
- Compare your answer with your answer to part (a). [3]
The company also manufactures 'Ultrapower' batteries, which are stated to have a mean lifetime of 210 minutes.
- A random sample of 8 Ultrapower batteries is selected. The mean lifetime of these batteries is 207.3 minutes.
Carry out a hypothesis test at the 5% level to investigate whether the mean lifetime is as high as stated. You should use the following hypotheses \(\text{H}_0 : \mu = 210\), \(\text{H}_1 : \mu < 210\), where \(\mu\) represents the population mean for Ultrapower batteries.
You should assume that the population is Normally distributed with standard deviation 3.4. [5]