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    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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    Please post your working to illustrate where the minus sign is an issue.
 
 
 
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Updated: October 6, 2017
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