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# s1 qn watch

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1. This qn is takenfrom last june's s1 edexcel paper.

A health club l ets members use, on each visit, its facilities for as long as they wish. The club's records suggest that the length of a visit can be modelled by a normal distribution with mean 90 minutes. Only 20% of members stay for more than 125 minutes.

Find the standard deviation of the normal distribution.

The answer is 41.6 - I can start the qn off but then get stuck. Can anyone please post a solution.

Thank you
2. (Original post by Jackal123)
This qn is takenfrom last june's s1 edexcel paper.

A health club l ets members use, on each visit, its facilities for as long as they wish. The club's records suggest that the length of a visit can be modelled by a normal distribution with mean 90 minutes. Only 20% of members stay for more than 125 minutes.

Find the standard deviation of the normal distribution.

The answer is 41.6 - I can start the qn off but then get stuck. Can anyone please post a solution.

Thank you
Think for a minute.
I'm not going to post a solution for you. But you do have some useful facts here.

20 % of members have time t > 125 minutes. IE P(t>125) = 0.2
Phi(z) = 0.8 (area to the left of 125), and in tables you'll get a z-value.

Use the formula for z = ...... using the mean, and the sample value and the z-value that we know, and the only variable you are missing from the formula is the one you are after.

Rearrange the formula to make your variable the subject of the formula.

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