Combining expected values and variances

slowrolla

New Member
Hi! First post here so cut me a bit of slack Hopefully an easy topic too (I'm new to stats).

If I have some values for E(X) (e.g. 5min) and Var(X) (e.g. 2min^2) for a uniform distribution for say, the waiting time at a bank, how can I calculate, for example, the E(X) and Var(X) over a week (where I visit the bank 5 times)?
I know there are rules that state E(cX)=cE(X) and Var(cX)=c^2Var(X), though I don't think they apply here. I think here, we are dealing with summations of E(X) and Var(X). Is it just possible to add them up? For example, can I just say that the expected value for waiting time over the course of the week is the sum of the waiting times for each day (25min)? I know this will give me the same result as using E(cX)=cE(X), but it is different for variance. For variance, can I just add up the variances to get the variance for waiting time over the course of the week (10min^2)? If I used Var(cX)=c^2Var(X), I would get 50min^2.

Please keep the answers as non-technical as possible 