The life of a particular type of torch battery is normally distributed with mean 120 hours and standard deviation s hours. It is known that 87.5% of these batteries last longer than 70 hours.
Find the value of s.
Also please make it clear about where to use the minus sign and where not to. I think it is used in this question as well. I am really confused on this part. Thank you.
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Last edited by kundanad; 06-10-2017 at 06:18.
- 06-10-2017 06:16
- 06-10-2017 08:19
Please post your working to illustrate where the minus sign is an issue.