Number to words challenge

number_to_word.rb

def find_unit(num, hash)
  unit = hash.select { |key, _value| key <= num }
  unit.keys.last
end

def num_to_words(num)
  numbers_to_name = {
    1 => 'one',
    2 => 'two',
    3 => 'three',
    4 => 'four',
    5 => 'five',
    6 => 'six',
    7 => 'seven',
    8 => 'eight',
    9 => 'nine',
    10 => 'ten',
    11 => 'eleven',
    12 => 'twelve',
    13 => 'thirteen',
    14 => 'fourteen',
    15 => 'fifteen',
    16 => 'sixteen',
    17 => 'seventeen',
    18 => 'eighteen',
    19 => 'nineteen',
    20 => 'twenty',
    30 => 'thirty',
    40 => 'fourty',
    50 => 'fifty',
    60 => 'sixty',
    70 => 'seventy',
    80 => 'eighty',
    90 => 'ninety',
    100 => 'hundred',
    1000 => 'thousand'
  }

  return 'zero' if num.zero?

  string = ''

  while num > 0
    unit = find_unit(num, numbers_to_name)
    unit_name = numbers_to_name[unit]

    if num == unit && num <= 100
      string += " #{unit_name}"
      return string
    end

    result = num / unit
    string += if string.empty? && num >= 100
                "#{numbers_to_name[result]} #{unit_name}"
              else
                " #{unit_name}"
              end

    modulo = num % unit

    break if modulo.zero?

    num = modulo
  end

  string
end

puts num_to_words(3)
puts num_to_words(103)
puts num_to_words(131)