diff --git a/20201216/main.go b/20201216/main.go
new file mode 100644
index 0000000..42d3acc
--- /dev/null
+++ b/20201216/main.go
@@ -0,0 +1,31 @@
+package main
+
+import "fmt"
+
+func main() {
+
+}
+
+func process(n int) []string {
+	m := make(map[string]struct{})
+	for i := 2; i <= n; i++ {
+		for j := 1; j < i; j++ {
+			g := findGCD(i, j)
+			m[fmt.Sprintf("%d/%d", j/g, i/g)] = struct{}{}
+		}
+	}
+	var ret []string
+	for k := range m {
+		ret = append(ret, k)
+	}
+	return ret
+}
+
+func findGCD(a, b int) int {
+	for a > 0 {
+		t := a
+		a = b % a
+		b = t
+	}
+	return b
+}
diff --git a/20201216/main_test.go b/20201216/main_test.go
new file mode 100644
index 0000000..9c0c839
--- /dev/null
+++ b/20201216/main_test.go
@@ -0,0 +1,63 @@
+package main
+
+import (
+	"reflect"
+	"sort"
+	"testing"
+)
+
+func Test_process(t *testing.T) {
+	type args struct {
+		n int
+	}
+	tests := []struct {
+		name string
+		args args
+		want []string
+	}{
+		{
+			"test1",
+			args{
+				n: 2,
+			},
+			[]string{"1/2"},
+		},
+		{
+			"test2",
+			args{
+				n: 3,
+			},
+			[]string{"1/2", "1/3", "2/3"},
+		},
+		{
+			"test3",
+			args{
+				n: 4,
+			},
+			[]string{"1/2", "1/3", "1/4", "2/3", "3/4"},
+		},
+		{
+			"test4",
+			args{
+				n: 1,
+			},
+			nil,
+		},
+		{
+			"test5",
+			args{
+				n: 9,
+			},
+			[]string{"1/2", "1/3", "1/4", "1/5", "1/6", "1/7", "1/8", "1/9", "2/3", "2/5", "2/7", "2/9", "3/4", "3/5", "3/7", "3/8", "4/5", "4/7", "4/9", "5/6", "5/7", "5/8", "5/9", "6/7", "7/8", "7/9", "8/9"},
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := process(tt.args.n)
+			sort.Strings(got)
+			if !reflect.DeepEqual(got, tt.want) {
+				t.Errorf("process() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}