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###### Help normal distribution maths q

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1 year ago

Help with this question I don’t get it I got these for the z values

(edited 1 year ago)

Reply 2

1 year ago

Original post by vc94

Is your sigma value correct...?

I don’t know how to work it out

Reply 4

1 year ago

Original post by vc94

Your working has 1-Mu = -1.405 x sigma and 5-Mu= 1.0803 x sigma

Solve simultaneously for Mu and sigma.

But you got Mu=3.26 which I agree with, your sigma is wrong I believe.

Solve simultaneously for Mu and sigma.

But you got Mu=3.26 which I agree with, your sigma is wrong I believe.

The one on the whiteboard is my working the other one is the markscheme (sigma is 1.61 I wrote it wrong by accident from ms)

But my whiteboard method why is it incorrect ??

I get wrong answer

(edited 1 year ago)

Reply 6

1 year ago

Original post by vc94

Okay, so how did you get your 2 z-values?

Well,

Z~N(0,1^2)

For the p(x<1) it’s a left tail and 0.08 for the area and I put all that in my calculator (graphical) to get the value for Z

Same for the other z value

P(X<1) is a probability from the distribution X~N( Mu, variance) with the unknown parameters.

When you standardise X, you get

P(Z < (1 - mu) / sigma )) =0.08 which is from the distribution Z~N(0, 1^2) .... so you can use the inverse normal function on this which leads to

(1 - mu) / sigma = -1.405

When you standardise X, you get

P(Z < (1 - mu) / sigma )) =0.08 which is from the distribution Z~N(0, 1^2) .... so you can use the inverse normal function on this which leads to

(1 - mu) / sigma = -1.405

(edited 1 year ago)

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