Adjust position of the blanks

This way, the test framework can throw the correct errors for
this test as it only allows blanks to appear first.

Also, adjust the answer correspondingly.

koans-solved/let.lisp

@@ -17,14 +17,14 @@
   ;; created: a symbol that names a variable becomes bound to a value.
   (let ((x 10)
         (y 20))
-    (assert-equal (+ x y) 30)
+    (assert-equal 30 (+ x y))
     ;; It is possible to shadow previously visible bindings.
     (let ((y 30))
-      (assert-equal (+ x y) 40))
-    (assert-equal (+ x y) 30))
+      (assert-equal 40 (+ x y)))
+    (assert-equal 30 (+ x y)))
   ;; Variables bound by LET have a default value of NIL.
   (let (x)
-    (assert-equal x nil)))
+    (assert-equal nil x)))
 
 (define-test let-versus-let*
   ;; LET* is similar to LET, except the bindings are established sequentially,

koans/let.lisp

@@ -17,14 +17,14 @@
   ;; created: a symbol that names a variable becomes bound to a value.
   (let ((x 10)
         (y 20))
-    (assert-equal (+ x y) ____)
+    (assert-equal ____ (+ x y))
     ;; It is possible to shadow previously visible bindings.
     (let ((y 30))
-      (assert-equal (+ x y) ____))
-    (assert-equal (+ x y) ____))
+      (assert-equal ____ (+ x y)))
+    (assert-equal ____ (+ x y)))
   ;; Variables bound by LET have a default value of NIL.
   (let (x)
-    (assert-equal x ____)))
+    (assert-equal ____ x)))
 
 (define-test let-versus-let*
   ;; LET* is similar to LET, except the bindings are established sequentially,