Problem 20 turned out to be yet another ridiculously simple problem for Clojure. Here’s how to sum the digits in 100 factorial:

```
(defn factorial [n] (reduce * (range 2 (inc n))))
(defn sum-digits [s] (reduce + (map #(Integer/parseInt (str %)) s)))
(defn euler-20 [] (sum-digits (str (factorial 100))))
(euler-20)
```