Scheme 方案中的数据定义？

我今天一直在研究这个问题，剩下的工作就是开发一个函数，该函数接受服务员并返回服务员分配给的动物的总年龄。所以如果我拿服务员戴夫为例，我会得到10分。不知道从哪里开始。你怎么把年龄加起来

(define-struct animal (name species age breakfasthour dinnerhour))
    (define-struct attendant (name a1 a2 a3))


    (define gorilla (make-animal "Koko" "Gorilla" "4" "8" "10"))
    (define bat (make-animal "Bruce" "Bat" "1" "23" "5"))
    (define mandrill (make-animal "Manny" "Mandrill" "5" "8" "7"))
    (define crocodile (make-animal "Swampy" "Crocodile" "1" "10" "8"))
    (define ocelot (make-animal "Ozzy" "Ocelot" "7" "7" "17"))
    (define capybara (make-animal "Capy" "Capybara" "4" "6" "8"))
    (define potto (make-animal "Spot" "Potto" "2" "2" "6"))
    (define tapir (make-animal "Stripey" "Tapir" "3" "10" "6"))
    (define vulture (make-animal "Beaky" "Vulture" "10" "9" "6"))


    (define attendant1 (make-attendant "Dave" gorilla bat mandrill))
    (define attendant2 (make-attendant "John" crocodile ocelot capybara))
    (define attendant3 (make-attendant "Joe" potto tapir vulture))


    #;(define (meal-time? e1 e2)
      (string=? (animal-species e1)
                (animal-dinnerhour e2)))



    #;(define (animal-template s...)
      (...(animal-name s)...
          (animal-species s)...
          (animal-age s)...
          (animal-breakfasthour s)...
          (animal-dinnerhour s)...))

    #;(define (attendant-template s...)
      (...(attendant-name s)...
          (attendant-s1 s)...
          (attendant-s2 s)...
          (attendant-s3 s)...))

---

只需对每种类型使用访问器过程：

(define (animals-age att)
  (+ (animal-age (attendant-a1 att))
     (animal-age (attendant-a2 att))
     (animal-age (attendant-a3 att))))

---

很明显，只有当年龄是一个数字时，在当前代码中是一个字符串（为什么？），请考虑将年龄表示为数字，它更有意义。

< P>简单地使用每种类型的访问程序：

(define (animals-age att)
  (+ (animal-age (attendant-a1 att))
     (animal-age (attendant-a2 att))
     (animal-age (attendant-a3 att))))

很清楚，只有当年龄是一个数字时，在你当前的代码中是一个字符串（为什么？），请考虑把年龄表示为数字，这就更有意义了。