For example, if I am rolling two dice, one with 8 faces and the other with 10. How do I calculate...

...the odds of rolling a certain combo, say, a 1 and a 10,

...the odds of rolling a certain digit on either die, say a 4

...the odds of the total value, like "the odds of a total of 12", and finally,

...the average value of any given roll?

I'm designing a board game, and so I'm trying to figure some of these combos out.

I can figure the odds of each die, for an 8 and 10 sided, each face is 1/8. But I don't know how to combine odds... like the odds of rolling a 9 could be a 1 and an 8, 2 and a 7, etc. But I get confused with the odds of rolling a 1 on *either* die - they're different, b/c of the different # of faces.

Also average value, I have no clue where to begin.

Thank you!