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MSV 18: The Coffee Problem

www.making-statistics-vital.co.uk. MSV 18: The Coffee Problem. Sam’s cafe is doing well. His coffee machines are dispensing X ml of coffee into a standard cup, where X ~ N( a , b 2 ). The owner’s rule is that 98% of standard cups should contain more than c ml of coffee,

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MSV 18: The Coffee Problem

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  1. www.making-statistics-vital.co.uk MSV 18: The Coffee Problem

  2. Sam’s cafe is doing well. His coffee machines are dispensing X ml of coffee into a standard cup, where X ~ N(a, b2).

  3. The owner’s rule is that 98% of standard cups should contain more than c ml of coffee, and Sam’s machines obey this rule, just! Now Sam gets some worrying news.

  4. The owner revises their rule so that 99% of standard cups have to contain more than c ml of coffee. Sam does some calculations to find out how he has to adjust his machines.

  5. ‘If I increase the mean a by 1%, that will do the trick exactly...’ ‘But looking at it a different way, if I reduce the standard deviation b by 0.1, that will do what’s needed exactly as well!’ Can you find a, bandc? (Give your answers to 3sf.)

  6. Answers

  7. From 1 and 3, 2.054b = 2.326(b - 0.1) so b = 0.855ml. Subtracting 1 from 2, 0.01a = 0.2326, so a = 23.3ml. We now have c = 21.5ml.

  8. With thanks to Pixabay.com and to Sam Green. www.making-statistics-vital.co.uk is written by Jonny Griffiths hello@jonny-griffiths.net

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