diff --git a/day400/problem.go b/day400/problem.go
new file mode 100644
index 0000000..b6f05c0
--- /dev/null
+++ b/day400/problem.go
@@ -0,0 +1,13 @@
+package day149
+
+// SumSublist returns the sum of the sublist from start (inclusive)
+// up to end (exclusive).
+// Runs in O(end-start).
+func SumSublist(l []int, start, end int) int {
+	var sum int
+	for i := start; i < end; i++ {
+		sum += l[i]
+	}
+
+	return sum
+}
diff --git a/day400/problem_test.go b/day400/problem_test.go
new file mode 100644
index 0000000..6d4e64b
--- /dev/null
+++ b/day400/problem_test.go
@@ -0,0 +1,30 @@
+package day149
+
+import "testing"
+
+// nolint
+var testcases = []struct {
+	L          []int
+	start, end int
+	expected   int
+}{
+	{[]int{1, 2, 3, 4, 5}, 1, 3, 5},
+}
+
+func TestSumSublist(t *testing.T) {
+	t.Parallel()
+
+	for _, tc := range testcases {
+		if result := SumSublist(tc.L, tc.start, tc.end); result != tc.expected {
+			t.Errorf("Expected %v got %v", tc.expected, result)
+		}
+	}
+}
+
+func BenchmarkSumSublist(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		for _, tc := range testcases {
+			SumSublist(tc.L, tc.start, tc.end)
+		}
+	}
+}