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# Standard Deviation - Statistics Maths Edexcel

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1. Paper June 2007 Q2

It asks to find the standard deviation and this is the method in the marks scheme:
54-45 / SD = 0.67

So they used this formula:
3(mean-median) / SD = skew

But I don't understand how they got the skew and why didn't they put in 3(...) ?
2. They do not use the skewness formula.
As they say that the distribution is normally distributed you can use (X-mean)/standard deviation=Z.
As the data is normally distributed then median=mean.
You know that for the UQ X=54 and Z=0.75 as P(X<54)=0.75 Therefore rearrange the above equation to fine the standard deviation.

Hope this helps!

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