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Stuck on standard deviation question

I had to put it into 2 screenshots as theyre on different pages, so theres 2 attachments.

I worked out 1 standard dev below the mean and got 11.41 hours but I'm not sure what to do next, and the working out on the solutions is not clear to me.

Thanks :smile:
1.png18.4KB
2.png30.6KB
you need to estimate the number of parts which lasted longer than 11.41 hours.and see if that number is at least 90% of the total parts.
Original post by the bear
you need to estimate the number of parts which lasted longer than 11.41 hours.and see if that number is at least 90% of the total parts.


So should I do

10+(11.4-5/14) x 5 =12.

50-12=38 parts?

and since 38 < 45 (which is 90%) the claim is false?
( 11.41 - 10) / 5 is the proportion of the second group from the left which we need to consider... then multiply this by the number of people in that group ( which is 14 ).

then add the result onto 5... this will give you an estimate of the number of parts which last under 11.41 hours...

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