Question 773759
1) the probability that a call will take less that 5 minutes is equal to 60%. What is the probability that a call will take between 4.5 and 5 minutes

Pr(4.5<X<5) = Pr(X<5) - Pr(X>4.5)
by definition of the mean Pr(X>4.5) is 50%
Pr(4.5<X<5) = 60% - 50% = 10% 
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2) You are given that the a call will take more than 3 minutes is equal to 80%. What is the probability what a call will take more than 6 minutes. 
Pr(X>3) = 1 - Pr(X<3) = 80%  and Pr(X<3) = 20%
need to calculate std dev
z-value = (3 - 4.5) / std. dev
consult Z table for 20% probability = -.84
-.84 = -1.5 / std. dev 
std. dev = 1.5/.84 = 1.78
now
Pr(X>6) = 1 - Pr(X<6)
z-value = (6 -4.5) / 1.78 = .84
Pr(X<6) = 80%
Pr(X>6) = 1 - Pr(X<6)= 1 -.80 = 20%