diff --git a/Labs/05.rkt b/Labs/05.rkt
new file mode 100644
index 0000000..0866657
--- /dev/null
+++ b/Labs/05.rkt
@@ -0,0 +1,64 @@
+#lang plait
+(define-type Exp
+  [strE (v : String)]
+  [numE (v : Number)]
+  [prodE (l : Exp) (r : Exp)])
+
+(define-type Value
+  [numV (n : Number)]
+  [strV (s : String)])
+
+(define-type Type
+  [numT]
+  [strT])
+
+(define (times n str) (if (zero? n) "" (string-append str (times (sub1 n) str))))
+
+(define (interp slang)
+  (type-case Exp slang
+    [(strE s) (strV s)]
+    [(numE n) (numV n)]
+    [(prodE l r)
+     (let ([l-v (interp l)]
+           [r-v (interp r)])
+       (cond
+         [(and (numV? l-v) (numV? r-v))
+          (numV (* (numV-n l-v) (numV-n r-v)))]
+         [(numV? l-v)
+          (strV (times (numV-n l-v) (strV-s r-v)))]
+         [(numV? r-v)
+          (strV (times (numV-n r-v) (strV-s l-v)))]         
+         [else (error 'interp "type mismatch")]))]))
+
+(test (interp (prodE (numE 6) (numE 7))) (numV 42))
+(test (interp (prodE (numE 3) (strE "ha"))) (strV "hahaha"))
+(test (interp (prodE (numE 2) (prodE (numE 2) (strE "ha")))) (strV "hahahaha"))
+(test (interp (prodE (strE "hello") (numE 3))) (strV "hellohellohello"))
+(test/exn (interp (prodE (strE "hello") (strE "world"))) "mismatch")
+
+(define (typecheck exp)
+  (type-case Exp exp
+    [(strE v) (strT)]
+    [(numE v) (numT)]
+    [(prodE l r)
+     (let
+         ([lt (typecheck l)]
+          [rt (typecheck r)])
+       (cond
+         [(and (numT? lt) (numT? rt)) lt]
+         [(numT? lt) rt]
+         [(numT? rt) lt]
+         [else (error 'typecheck "type mismatch")]))]))
+
+(test (typecheck (numE 6)) (numT))
+(test (typecheck (strE "hello")) (strT))
+(test (typecheck (prodE (numE 6) (numE 7))) (numT))
+(test (typecheck (prodE (numE 3) (prodE (numE 6) (numE 7)))) (numT))
+(test (typecheck (prodE (numE 3) (strE "ha"))) (strT))
+(test (typecheck (prodE (strE "hello") (numE 3))) (strT))
+(test (typecheck (prodE (prodE (numE 3) (numE 6)) (numE 7))) (numT))
+(test/exn (typecheck (prodE (strE "hello") (strE "world"))) "mismatch")
+(test/exn (typecheck (prodE (prodE (numE 2) (strE "hello")) (strE "world"))) "mismatch")
+(test/exn (typecheck (prodE (prodE (numE 2) (strE "hello")) (prodE (strE "world") (numE 3)))) "mismatch")
+(test/exn (typecheck (prodE (strE "3") (prodE (numE 6) (strE "7")))) "mismatch")
+(test/exn (typecheck (prodE (strE "3") (prodE (strE "6") (strE "7")))) "mismatch")
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