Sum of numbers that equal 100
This is built assuming you want no duplicates in the results, since the goal is a little unclear.
Quickly Finding Combinations
The function you should be using is Array.combination(n), which generates an enumerator that yields all combinations of size n from the array.  Once you have that it is a trivial matter to only select the combinations that sum to the target value.
My function here finds all combinations of size 2 to size data.length and then selects only the combinations with the correct sum.
def find_sum(data, target_sum)
  (2..data.length)
    .flat_map { |n| data.combination(n).to_a }
    .select { |arr| arr.inject(:+) == target_sum }
    #.uniq   # can call this to remove duplicates if necessary, depending on what the input data looks like
end

data = (1..9).to_a
pp find_sum(data, 10)
#[[1, 9],
# [2, 8],
# [3, 7],
# [4, 6],
# [1, 2, 7],
# [1, 3, 6],
# [1, 4, 5],
# [2, 3, 5],
# [1, 2, 3, 4]]