Question 1160812
a. E(X)=50 minutes, same as individual average
SEM=10/sqrt(30)=1.826 minutes

z=(x-mean)/sd/sqrt(n)
> 5*sqrt(30)10=2.74
prob. z>2.74 is 0.0031

this would be z between -.5 and +.5 which is 0.3829

Much more difficult for the average of 30 to differ greatly from the mean than a single person.