marginal probability

Hi guys, if someone can help me with this homework i will aprecciated very much. I try to do the first one, but i only got de U=X and the V=X+Y. I have no idea of how to do the W=X+Y+Z, and therefore the points 2 and 3.

Dear talkstats-community, I already spent hours on websearch for an answer on a question I have regarding the interpretation of marginal effects (at means) of probit regressions. How can I explicitly interpret a marginal effect of a variable that enters in 1) logarithmized form (eg...

a very, very nice property of the multivariate normal distribution is that any lower-dimensional margins are also multivariate (or univariate, depending on how low you go) normally distributed. does anyone know any other distributions for which this property is true? is there some sort of...