hi prof eddy LECTURE 14..when you explained on the PREVENTIVE MAINTENANCE OF FREQUENCY, you gave us two examples.. on the second example its " suppose the time before breakdown is normally distributed with 3 weeks and standard deviation of ,60 weeks .If breakdown costs averages $1000 , and preventive maintenance costs $250 , what is the optmal maintenance interval ? we got ; prevent cost/ breakdown cost......= $250/$1000 =2.5 so where did you get this (-0,67) when you said you got in USING STANDARD NORMAL TABLE.. what is really this standard normal table ? thanks Mr eddy
