Question 1081259
z=(x-mean)/sd for all of these
for 4.2 and 4.9
z=0/0.4=0 to (4.9-4.2)/0.4=0.7/0.4=1.75
probability from z between 0 and +1.75 is 0.4599.
-----------------------
More than 4.9 minutes: We know that more than 4.2 is 0.5 and between 4.2 and 4.9 is 0.4599.  Subtract the second from the first and get the value of greater than 4.9, which 0.0401.  It is z> 1.75
-----------------------
Between 4.9 and 5.5 minutes is z between +1.75 and 1.3/0.4=3.25, The probability is 0.0395.
----------------------
Between 3.5 and 5.5 minutes is z between -0.7/0.4=-1.75 and +3.25.  That is 0.9594.
---------------------
The longest 6% of calls is where the probability is 0.9400.  This is a z of +1.55.
From the top equation, z*sd=x-mean=1.55*0.4=.62=x-mean; x=4.2+0.62=4.82 minutes or 4m49s