An unfair 12-sided die X (because 1-8 come up 10% of the time each and 9-12 5% each) has mean 5.7 and standard deviation 3.195, and a fair 20-sided die Y has mean 10.5 and standard deviation 5.766. (You can check these if you're interested, but use the rounded versions given here.) We roll both dice. Let S be the sum. Find the mean of S.

Can anyone show me the formula/explain how to do problems like this?

Just add the means right?

independent rv

mean you just add

same with variance

So my answer is 11.5 rounded to one decimal? that simple?

should be 5.7 + 10.5

since the mean sum is equal to the sum of the means.

just think of it this way: the mean one dice is 5.7 and the other is 10.5. now all you are doing is summing the die rolls and by doing this, you affect the outcome of the individual rolls what so ever. You would except now the mean of the sum to just be the sum of the means.