DONT + TAKE = MONEY; M=Y, A=D

wordsum.rb

def generate_map(letters, numbers, map, words)
  return test_sum(map, words) if letters.empty?

  letter = letters[0]
  numbers.each do |number|
    generate_map(
      letters - [letter],
      numbers - [number],
      map.merge(letter => number),
      words
    )
  end
end

def test_sum(map, words)
  numbers = []
  words.each do |word|
    number = 0
    word.each_char do |c|
      number = number * 10 + map[c]
    end
    numbers << number
  end

  puts "#{numbers[0]} + #{numbers[1]} = #{numbers[2]}" if (numbers[0] + numbers[1]) == numbers[2]
end

words = ["DONT", "TDKE", "MONEM"]
letters = words.join("").split("").uniq
letters.delete("M")
generate_map(letters, (2..9).to_a, { "M" => 1 }, words)