Question 835719
1.{{{z=(x-m)/sigma=(15-18)/2=-3/2}}}
{{{P(-3/2)=0.0668}}}
So the probability it lasts longer than 15 hours is,
{{{P=1-0.0668=0.9332}}}
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2..{{{z=(x-m)/sigma=(16-18)/2=-2/2=1}}}
{{{P(-1)=0.158655}}}
15.8655% would fail by 16 hours so 84.135% would survive.
{{{1000/X=.84135}}}
{{{X=1188.573}}}
You should buy 1189 batteries.