numbers   = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
operators = ["+", "-", ""]

operations = numbers.size - 1

expressions = operators.repeated_permutation(operations).map do |sequence|
  numbers.zip(sequence).join
end

p "Exploring #{expressions.size} possibilities."

hit = expressions.find { |exp| eval(exp) == 100 }

p "Found! #{hit} == 100"