電車の広告で見かけたので解いてみた。
逆ポーランド記法を用いた。
簡易な電卓機能を作れるかを試すような問題だった。
例は1,3,3,7の場合。

class Calculator
  OPERATORS = %w[+ - * /]

  def exec
    answer_formulas = []
    OPERATORS.each do |l|
      first_ope = l
      OPERATORS.each do |m|
        second_ope = m
        OPERATORS.each do |n|
          third_ope = n
          [1.0, 3.0, 3.0, 7.0, first_ope, second_ope, third_ope].permutation.to_a.each do |formula|
            result = _calc(formula.dup)
            if result == 10.0
              answer_formulas << formula
            end
          end
        end
      end
    end
    p answer_formulas.uniq
    p answer_formulas.uniq.count
  end

  private

  def _calc(formula)
    stack = []
    while formula.size > 0
      item = formula.shift
      if OPERATORS.include? item
        if stack.size < 2 || OPERATORS.include?(stack[-1]) || OPERATORS.include?(stack[-2])
          stack.unshift "failure"
          break
        end

        case item
        when '+'
          right = stack.pop
          left = stack.pop
          stack << left + right
        when '-'
          right = stack.pop
          left = stack.pop
          stack << left - right
        when '*'
          right = stack.pop
          left = stack.pop
          stack << left * right
        when '/'
          right = stack.pop
          left = stack.pop
          stack << left / right
        end
      else
        stack << item
      end
    end
    stack[0]
  end
end

Calculator.new.exec