Overall there is no increase or decrease in her account
Neither, for lines to be parallel they have to have the same slope, and for lines to be perpendicular they have to be the negative reciprocal of the slope of the other line. In this case, the line perpendicular to the line with slope m=10 would be m=1/-10 or -1/10
a. Let 
be a random variable representing the weight of a ball bearing selected at random. We're told that 
, so
where 
. This probability is approximately
b. Let 
be a random variable representing the weight of the 
-th ball that is selected, and let 
be the mean of these 4 weights,
The sum of normally distributed random variables is a random variable that also follows a normal distribution,
so that
Then
c. Same as (b).
