diff --git a/20_factorial_digit_sum/factorial_test.go b/20_factorial_digit_sum/factorial_test.go
new file mode 100644
index 0000000..abb860a
--- /dev/null
+++ b/20_factorial_digit_sum/factorial_test.go
@@ -0,0 +1,76 @@
+package main
+
+import (
+	"testing"
+)
+
+func Test_findFactorial(t *testing.T) {
+	tt := []struct {
+		name  string
+		input int
+		want  string
+	}{
+		{
+			"2",
+			2,
+			"2",
+		},
+		{
+			"4",
+			4,
+			"24",
+		},
+		{
+			"10",
+			10,
+			"3628800",
+		},
+	}
+
+	for _, tc := range tt {
+		t.Run(tc.name, func(t *testing.T) {
+			got := findFactorial(tc.input)
+			if got != tc.want {
+				t.Errorf("findFactorial=%v, want %v", got, tc.want)
+			}
+		})
+	}
+}
+
+func Test_sumDigits(t *testing.T) {
+	tt := []struct {
+		name  string
+		input string
+		want  int
+	}{
+		{
+			"no_text",
+			"",
+			0,
+		},
+		{
+			"1",
+			"1",
+			1,
+		},
+		{
+			"12345",
+			"12345",
+			15,
+		},
+		{
+			"3628800",
+			"3628800",
+			27,
+		},
+	}
+
+	for _, tc := range tt {
+		t.Run(tc.name, func(t *testing.T) {
+			got := sumDigits(tc.input)
+			if got != tc.want {
+				t.Errorf("sumDigits = %d, want %d", got, tc.want)
+			}
+		})
+	}
+}
diff --git a/20_factorial_digit_sum/main.go b/20_factorial_digit_sum/main.go
new file mode 100644
index 0000000..086e2e5
--- /dev/null
+++ b/20_factorial_digit_sum/main.go
@@ -0,0 +1,38 @@
+package main
+
+import (
+	"fmt"
+	"math/big"
+	"strconv"
+)
+
+func main() {
+	fmt.Println("sum:", sumDigits(findFactorial(100)))
+}
+
+func findFactorial(input int) string {
+	product, _ := (&big.Int{}).SetString("1", 0)
+	for input > 0 {
+		i, _ := (&big.Int{}).SetString(strconv.Itoa(input), 0)
+		product = product.Mul(product, i)
+		input--
+	}
+	return product.String()
+}
+
+func sumDigits(input string) int {
+	l := len(input)
+	if l == 0 {
+		return 0
+	}
+	sum := 0
+	for i := 0; i < l; i++ {
+		ns := input[i : i+1]
+		n, err := strconv.Atoi(ns)
+		if err != nil {
+			panic(err)
+		}
+		sum += n
+	}
+	return sum
+}