Statistics help (as level) ASAp

[regmimanish]

The lengths, in centimeter of drinking straws produced in factory have a normal distribution with mean (M) and varience 0.64 . It is given that 10 % of the straws are shorter than 20 cm .


I) Find the value of M
2 : ) Find the probability that , of 4 straws chosen at random , fewer than 2 will have length between 21.5 and 22.5

 

[destined007]

Here:

 

[regmimanish]

why u did 1 - 0. 1 ?? isn't that the probility for p X>20
and also how did you get -1.282 on the 3rd step plz help i didn't get the sooln

 

[destined007]