Is there a formula to calculate the theoretical probabilities of the sums of two die?

Question

I need to find the theoretical prob. of each sum of two 20 sided die and 3 six sided die. Is there a formula for this, because I can't find any. PLEASE HELP

Answer 1

the best way is to make a tabulation (for three conventional cube-shaped dice) showing all possible choices. E.g.:
D1 D2 D3
1 1 1
1 1 2, etc.
There will be 63 entries (216) and you can make a fourth column showing totals.
P(total 3) will be 1/216
P(total 4) will be 3/216 because it requires two 1s and one 2, and that 2 can be in positions D2, D2 or D3.