smallest_zero_one.rb

class Fixnum
  def is_zero_one?
    self.to_s.chars.all? { |e| '01'.include? e }
  end

  def smallest_zero_one
    unless (1...100_000).include?(self)
      fail ArgumentError.new('Integer should be between 1 and 99,999')
    end

    self * (1..Float::INFINITY).find { |c| (self * c).is_zero_one? }
  end
end

puts 4.smallest_zero_one
puts 10.smallest_zero_one
puts 40.smallest_zero_one
puts 101.smallest_zero_one
puts 100_000.smallest_zero_one