Probability and average in dice rolls

[TeufelDK]

Hi

I need a bit of help calculating a specific dice rolling problem:

Using 10-sides dice, I need help calculating the average of Y dice rolls where you count the X highest or the X lowest dice.
So X/Y means: roll Y dice and add the X highest. -X/Y means: roll Y dice and add the X lowest dices
So 2/5 would mean: "roll 5 dice and add the 2 highest". -2/6 would mean rolls 6 dice and add the 2 lowest.

How do I calculate the average of an X/Y roll? How can you calculate the probability of getting for instance 5 or higher on an X/Y roll or and -X/Y roll?

I hope you can help, and that I have explained the problem in an understandable way.
All the best, and thank you for your time.

Regards
Kristian

 

[Steven G]

Yes, we can help but you first need to follow the posting guidelines by showing us your work and where you get stuck. This is after all a math help forum and not a homework site. So please read the posting guidelines, post back following them and you'll receive expert help.