stats paper tomorrow help please

[weesha]

i cant understand part ii of q7 variant 62 nov 09
The weights, X grams, of bars of soap are normally distributed with mean 125 grams and standard
deviation 4.2 grams.
(i) Find the probability that a randomly chosen bar of soap weighs more than 128 grams.
ans=0.238
(ii) Find the value of k such that P(k < X < 128) = 0.7465.
explain plz :?

 

[khizarr]

P(X>128)
1 - P( Z <[128-125]/4.2)
1 - P( Z <3/4.2)
find the value of (3/4.2) from the table and then subtract it from 1 , u'll have the answer!

(ii) P( [k-125]/4.2 < Z < 3/4.2) = 0.7465
P(Z<3/4.2) - P(Z < [k-125]/4.2) = 0.7465

the prob. which u will find in the previous part, subtract it from 0.7465. then find the value of that prob. from the table , inverse thing, the equate it to (k-125)/4.2 , uwill get the answer

 

[weesha]

thanks alot

 

[khoshi]

Should jt bs z>k-125/4.2???

 

[Kangxidi]

Should jt bs z>k-125/4.2???

Yes it should be

-Sorry for the random dropping by, would love to join in P6 discussion as well-