How do I get a list of all possible sums in a list nested list?

Question

Lets say I have the following input:

rn={
 {1, 1.1, 1.5},
 {5, 6, 6.1, 7},
 {8, 8, 12, 12, 12.5, 13}
}

This input has the dimensions of {3, 4, 6}.

I want to get each of the elements from each of the three lists, and add them to each of the elements of another list, for all possibilities, such as I would get a list that is 3 * 4 * 6 = 72 long.

I know that I can do this manually such that:

{
 rn[[1, 1]] + rn[[2, 1]] + rn[[3, 1]],
 rn[[1, 1]] + rn[[2, 1]] + rn[[3, 2]],
 rn[[1, 1]] + rn[[2, 1]] + rn[[3, 3]],
 rn[[1, 1]] + rn[[2, 1]] + rn[[3, 4]],
 rn[[1, 1]] + rn[[2, 1]] + rn[[3, 5]],
 rn[[1, 1]] + rn[[2, 1]] + rn[[3, 6]],
 rn[[1, 1]] + rn[[2, 2]] + rn[[3, 1]],
 rn[[1, 1]] + rn[[2, 2]] + rn[[3, 2]],
 rn[[1, 1]] + rn[[2, 2]] + rn[[3, 3]],
 rn[[1, 1]] + rn[[2, 2]] + rn[[3, 4]],
 rn[[1, 1]] + rn[[2, 2]] + rn[[3, 5]],
 rn[[1, 1]] + rn[[2, 2]] + rn[[3, 6]],
 rn[[1, 1]] + rn[[2, 3]] + rn[[3, 1]],
 rn[[1, 1]] + rn[[2, 3]] + rn[[3, 2]],
 rn[[1, 1]] + rn[[2, 3]] + rn[[3, 3]],
 rn[[1, 1]] + rn[[2, 3]] + rn[[3, 4]],
 rn[[1, 1]] + rn[[2, 3]] + rn[[3, 5]],
 rn[[1, 1]] + rn[[2, 3]] + rn[[3, 6]],
 rn[[1, 1]] + rn[[2, 4]] + rn[[3, 1]],
 rn[[1, 1]] + rn[[2, 4]] + rn[[3, 2]],
 rn[[1, 1]] + rn[[2, 4]] + rn[[3, 3]],
 rn[[1, 1]] + rn[[2, 4]] + rn[[3, 4]],
 rn[[1, 1]] + rn[[2, 4]] + rn[[3, 5]],
 rn[[1, 1]] + rn[[2, 4]] + rn[[3, 6]],
 rn[[1, 2]] + rn[[2, 1]] + rn[[3, 1]],
 rn[[1, 2]] + rn[[2, 1]] + rn[[3, 2]],
 rn[[1, 2]] + rn[[2, 1]] + rn[[3, 3]],
 rn[[1, 2]] + rn[[2, 1]] + rn[[3, 4]],
 rn[[1, 2]] + rn[[2, 1]] + rn[[3, 5]],
 rn[[1, 2]] + rn[[2, 1]] + rn[[3, 6]],
 rn[[1, 2]] + rn[[2, 2]] + rn[[3, 1]],
 rn[[1, 2]] + rn[[2, 2]] + rn[[3, 2]],
 rn[[1, 2]] + rn[[2, 2]] + rn[[3, 3]],
 rn[[1, 2]] + rn[[2, 2]] + rn[[3, 4]],
 rn[[1, 2]] + rn[[2, 2]] + rn[[3, 5]],
 rn[[1, 2]] + rn[[2, 2]] + rn[[3, 6]],
 rn[[1, 2]] + rn[[2, 3]] + rn[[3, 1]],
 rn[[1, 2]] + rn[[2, 3]] + rn[[3, 2]],
 rn[[1, 2]] + rn[[2, 3]] + rn[[3, 3]],
 rn[[1, 2]] + rn[[2, 3]] + rn[[3, 4]],
 rn[[1, 2]] + rn[[2, 3]] + rn[[3, 5]],
 rn[[1, 2]] + rn[[2, 3]] + rn[[3, 6]],
 rn[[1, 2]] + rn[[2, 4]] + rn[[3, 1]],
 rn[[1, 2]] + rn[[2, 4]] + rn[[3, 2]],
 rn[[1, 2]] + rn[[2, 4]] + rn[[3, 3]],
 rn[[1, 2]] + rn[[2, 4]] + rn[[3, 4]],
 rn[[1, 2]] + rn[[2, 4]] + rn[[3, 5]],
 rn[[1, 2]] + rn[[2, 4]] + rn[[3, 6]],
 rn[[1, 3]] + rn[[2, 1]] + rn[[3, 1]],
 rn[[1, 3]] + rn[[2, 1]] + rn[[3, 2]],
 rn[[1, 3]] + rn[[2, 1]] + rn[[3, 3]],
 rn[[1, 3]] + rn[[2, 1]] + rn[[3, 4]],
 rn[[1, 3]] + rn[[2, 1]] + rn[[3, 5]],
 rn[[1, 3]] + rn[[2, 1]] + rn[[3, 6]],
 rn[[1, 3]] + rn[[2, 2]] + rn[[3, 1]],
 rn[[1, 3]] + rn[[2, 2]] + rn[[3, 2]],
 rn[[1, 3]] + rn[[2, 2]] + rn[[3, 3]],
 rn[[1, 3]] + rn[[2, 2]] + rn[[3, 4]],
 rn[[1, 3]] + rn[[2, 2]] + rn[[3, 5]],
 rn[[1, 3]] + rn[[2, 2]] + rn[[3, 6]],
 rn[[1, 3]] + rn[[2, 3]] + rn[[3, 1]],
 rn[[1, 3]] + rn[[2, 3]] + rn[[3, 2]],
 rn[[1, 3]] + rn[[2, 3]] + rn[[3, 3]],
 rn[[1, 3]] + rn[[2, 3]] + rn[[3, 4]],
 rn[[1, 3]] + rn[[2, 3]] + rn[[3, 5]],
 rn[[1, 3]] + rn[[2, 3]] + rn[[3, 6]],
 rn[[1, 3]] + rn[[2, 4]] + rn[[3, 1]],
 rn[[1, 3]] + rn[[2, 4]] + rn[[3, 2]],
 rn[[1, 3]] + rn[[2, 4]] + rn[[3, 3]],
 rn[[1, 3]] + rn[[2, 4]] + rn[[3, 4]],
 rn[[1, 3]] + rn[[2, 4]] + rn[[3, 5]],
 rn[[1, 3]] + rn[[2, 4]] + rn[[3, 6]]
 }

gives the answer:

{14, 14, 18, 18, 18.5, 19, 15, 15, 19, 19, 19.5, 20, 15.1, 15.1, \
19.1, 19.1, 19.6, 20.1, 16, 16, 20, 20, 20.5, 21, 14.1, 14.1, 18.1, \
18.1, 18.6, 19.1, 15.1, 15.1, 19.1, 19.1, 19.6, 20.1, 15.2, 15.2, \
19.2, 19.2, 19.7, 20.2, 16.1, 16.1, 20.1, 20.1, 20.6, 21.1, 14.5, \
14.5, 18.5, 18.5, 19., 19.5, 15.5, 15.5, 19.5, 19.5, 20., 20.5, 15.6, \
15.6, 19.6, 19.6, 20.1, 20.6, 16.5, 16.5, 20.5, 20.5, 21., 21.5}

But there must be an easier way to do this using something like Tuples[], Table[], Sum[], Permutations[] so that I can give it a list that has any number of sub-lists, and sub lists that are of any length.

Answer 1

Total[Tuples@rn, {2}]

should do.

For cases where a very large number of tuples would be generated,

Fold[Total[Tuples[{##}], {2}] &, rn]

will generally be even quicker, and exhibit considerably lower memory pressure, to the point that it can be an order of magnitude faster because it can avoid paging if RAM gets low. If the ordering of the output is not important, sorting your $rn$ so that smaller lists are first (just a simple rn=Sort@rn) will increase the latter's performance even more.

Answer 2

Few additional alternatives:

Distribute[foo @@ rn, List, foo, List, Plus]

Flatten @ Outer[Plus, ## & @@ rn]

Activate @ Tuples[Inactive[Plus] @@ rn]

Answer 3

I see kglr got here first, but in a different format:

Outer[Plus, Apply[Sequence, rn]] // TableForm