gistfile1.clj

(def denoms [[:twenty 20]
             [:ten 10]
             [:five 5]
             [:dollar 1]
             [:quarter 1/4]
             [:dime 1/10]
             [:nickel 1/20]
             [:penny 1/100]])

(defn change-denom [denom amount]
  (let [[label size] denom
        [num remaining] ((juxt quot rem) amount size)]
    (into {:remainder remaining}
          (when-not (zero? num)
            {label num}))))

(defn make-change [amount]
  (dissoc (apply merge
                 (reductions (fn [{:keys [remainder]} denom]
                               (change-denom denom remainder))
                             {:remainder amount}
                             denoms))
          :remainder))

;; or

(defn make-change [amount]
  (-> {:remainder amount}
      (reductions (fn [{:keys [remainder]} denom]
                    (change-denom denom remainder))
                  denoms)
      (->> (apply merge))
      (dissoc :remainder)))