Doubts with the solution about proportional probability

[JeanBidu]

Could someone explain this last step to me? I didn't quite understand the ending, in fact a part yes, in the case -0.63 is based on the normal table Z

He wants to know the probability that the proportion is less than 75%

 

[Harry_the_cat]

You haven't given the whole question, so I'm assuming you are given n=25.

When p-hat=0.75, z=(0.75 - 0.80)/0.08 = -0.625 (Note that sqrt(0.0064)=0.08)

If you draw a diagram the probability you want is the part shaded under the normal curve to the left of x=-0.625.

This is 0.2660 approx. (Bit different to your answer because I use the exact value of -0.625 rather than -0.63)

 

[JeanBidu]

You haven't given the whole question, so I'm assuming you are given n=25.

When p-hat=0.75, z=(0.75 - 0.80)/0.08 = -0.625 (Note that sqrt(0.0064)=0.08)

If you draw a diagram the probability you want is the part shaded under the normal curve to the left of x=-0.625.

This is 0.2660 approx. (Bit different to your answer because I use the exact value of -0.625 rather than -0.63)

Yes, thank you very much for your answer, it helped me to remove this doubt, in the books I took to study there was no such explanation and I was always confused. Thank you very much

 

[Harry_the_cat]

Yes, thank you very much for your answer, it helped me to remove this doubt, in the books I took to study there was no such explanation and I was always confused. Thank you very much

I should have said ..."to the left of z=-0.625". ( ie z not x)