Both ques relates to sum of independent normal random variables. 1.) in q6.30, variance is found by 2^2 * 1 + (-1)^2 * 1 = 5 2.) but in other ques. on pg 37, variance is found by 3*90000 + 4*10000 = 310k 3.) why in (2.) variance isn't found by 3^2 *90000 + 4^2 *10000 = 970k
I have a feeling you are trying to memorise the formula which is not a bad thing provided you understand what is going on. I would recommend looking at Q6.25 and its solution. This will answer your question.
