(defn calc-coins [amount denom]
  {:mod (mod amount denom),
   :dividend (int (/ amount denom))})

(defn clos [amount]
  ; denoms is a list containing the currency denominations
  (let [denoms '(20 15)]
    (reduce
     (fn [[amount-left coins] denom]
       (let [{:keys [mod dividend]} (calc-coins amount-left denom)]
         (if (pos? dividend)
           [mod (assoc coins denom dividend)]
           [amount-left coins])))
     [amount {}]
     denoms)))