diff --git a/practice/040/answer/impl.ml b/practice/040/answer/impl.ml
new file mode 100644
index 0000000000..5bcd6adeef
--- /dev/null
+++ b/practice/040/answer/impl.ml
@@ -0,0 +1,11 @@
+let is_prime n =
+  let n = abs n in
+  let rec is_not_divisor d =
+    d * d > n || (n mod d <> 0 && is_not_divisor (d + 1)) in
+  n <> 1 && is_not_divisor 2
+
+let goldbach n =
+  let rec aux d =
+    if is_prime d && is_prime (n - d) then (d, n - d)
+    else aux (d + 1)
+  in aux 2
diff --git a/practice/040/answer/test/dune b/practice/040/answer/test/dune
new file mode 100644
index 0000000000..efa4aec676
--- /dev/null
+++ b/practice/040/answer/test/dune
@@ -0,0 +1,2 @@
+(include_subdirs no)
+(test (name run) (libraries ounit2 ex))
diff --git a/practice/040/answer/test/run.ml b/practice/040/answer/test/run.ml
new file mode 100644
index 0000000000..f415eab075
--- /dev/null
+++ b/practice/040/answer/test/run.ml
@@ -0,0 +1,3 @@
+module Test = Ex.Make(Ex.Answer)
+
+let () = OUnit2.run_test_tt_main Test.v
diff --git a/practice/040/dune b/practice/040/dune
new file mode 100644
index 0000000000..36077c968d
--- /dev/null
+++ b/practice/040/dune
@@ -0,0 +1,2 @@
+(include_subdirs qualified)
+(library (name ex) (libraries ounit2))
diff --git a/practice/040/dune-project b/practice/040/dune-project
new file mode 100644
index 0000000000..3c48133ad5
--- /dev/null
+++ b/practice/040/dune-project
@@ -0,0 +1 @@
+(lang dune 3.7)
diff --git a/practice/040/ex.ml b/practice/040/ex.ml
new file mode 100644
index 0000000000..eccfaf230a
--- /dev/null
+++ b/practice/040/ex.ml
@@ -0,0 +1,21 @@
+open OUnit2
+
+module type Testable = sig
+  val goldbach : int -> int * int
+end
+
+module Make(Tested: Testable) : sig val v : test end = struct
+  let tests = "Goldbach's conjecture" >::: [
+    "goldbach of 4" >:: (fun _ ->
+      assert_equal (2, 2) (Tested.goldbach 4));
+    "goldbach of 28" >:: (fun _ ->
+      assert_equal (5, 23) (Tested.goldbach 28));
+  ]
+
+  let v = "Tests for Goldbach's Conjecture" >::: [
+    tests
+  ]
+end
+
+module Work : Testable = Work.Impl
+module Answer : Testable = Answer.Impl
diff --git a/practice/040/work/impl.ml b/practice/040/work/impl.ml
new file mode 100644
index 0000000000..b1a9590611
--- /dev/null
+++ b/practice/040/work/impl.ml
@@ -0,0 +1 @@
+let goldbach _ = failwith "goldbach not implemented"
diff --git a/practice/040/work/test/dune b/practice/040/work/test/dune
new file mode 100644
index 0000000000..efa4aec676
--- /dev/null
+++ b/practice/040/work/test/dune
@@ -0,0 +1,2 @@
+(include_subdirs no)
+(test (name run) (libraries ounit2 ex))
diff --git a/practice/040/work/test/run.ml b/practice/040/work/test/run.ml
new file mode 100644
index 0000000000..7f63254fc5
--- /dev/null
+++ b/practice/040/work/test/run.ml
@@ -0,0 +1,3 @@
+module Test = Ex.Make(Ex.Work)
+
+let () = OUnit2.run_test_tt_main Test.v