I draw a standard normal curve and filled out everything but I couldn't figure it out. Can someone give me some hints?
ok. so mean and standard deviation are unknown. This guy spends less than one minute doing something about 40% of time and spends more than two minutes doing something 2% of time.
Any help would be appreciated.
Pr(Z < z1) = 0.4 => Pr(Z > -z1) = 0.4 => Pr(Z < -z1) = 0.6 at which point you go to your tables and get the value of -z1 and hence z1.
Since z1 is known to be negative before any calculation is done (why?) it might be easier to see it as:
Pr(Z < -a) = 0.4 => Pr(Z > a) = 0.4 => Pr(Z < a) = 0.6 and now go to tables. Then a = 0.2533 => -a = -0.2533.