diff --git a/ex-3_24-xx.scm b/ex-3_24-27.scm
similarity index 55%
rename from ex-3_24-xx.scm
rename to ex-3_24-27.scm
index 511527c..afca73c 100644
--- a/ex-3_24-xx.scm
+++ b/ex-3_24-27.scm
@@ -60,48 +60,72 @@
 
 (define (make-table same-key?)
   (define (assoc key records)
-    (display "assoc ") (display key) (display " ") (display records) (newline)
+    ; (display "assoc ") (display key) (display " ") (display records) (newline)
     (cond ((null? records) false)
           ((same-key? key (caar records)) (car records))
           (else (assoc key (cdr records)))))
   (let ((local-table (list '*table*)))
-    (define (lookup-generic . keys)
-      (display keys) (newline)
-      (lookup keys (cdr local-table)))
+
+    (define (lookup-proc . keys)
+      (lookup keys local-table))
     (define (lookup keys subtable)
-      (display "lookup ") (display keys) (display " ") (display subtable) (newline)
       (cond
         ((not subtable) #f)
         ((null? keys) (cdr subtable))
-        (else (lookup (cdr keys) (assoc (car keys) subtable)))))
-    (define (insert! key-1 key-2 value)
-      (let ((subtable (assoc key-1 (cdr local-table))))
-        (if subtable
-            (let ((record (assoc key-2 (cdr subtable))))
-              (if record
-                  (set-cdr! record value)
-                  (set-cdr! subtable
-                            (cons (cons key-2 value)
-                                  (cdr subtable)))))
-            (set-cdr! local-table
-                      (cons (list key-1
-                                  (cons key-2 value))
-                            (cdr local-table)))))
-      'ok)
+        (else (lookup (cdr keys) (assoc (car keys) (cdr subtable))))))
+
+    (define (insert!-proc . keys)
+      (insert! keys local-table))
+    (define (insert! keys subtable)
+      (if (null? (cddr keys))
+        (set-cdr! subtable
+                  (cons (cons (car keys) (cadr keys))
+                        (cdr subtable)))
+        (let ((existing-subtable (assoc (car keys) (cdr subtable))))
+          (if existing-subtable
+            (insert! (cdr keys) existing-subtable)
+            (let ((new-subtable (list (car keys))))
+              (set-cdr! subtable (cons new-subtable (cdr subtable)))
+              (insert! (cdr keys) new-subtable))))))
+
     (define (dispatch m)
-      (cond ((eq? m 'lookup-proc) lookup-generic)
-            ((eq? m 'insert-proc!) insert!)
+      (cond ((eq? m 'lookup-proc) lookup-proc)
+            ((eq? m 'insert-proc!) insert!-proc)
+            ((eq? m 'display-proc) (display local-table))
             (else (error "Unknown operation -- TABLE" m))))
     dispatch))
 
 (define operation-table (make-table equal?))
 (define get (operation-table 'lookup-proc))
 (define put (operation-table 'insert-proc!))
-(put 'a 'b 'c)
+
+(put 'a 'b)
+(put 'c 'd)
 (put 'd 'e 'f)
+(put 'd 'g 'h)
+(put 'x 'y 'z 'a)
+(assert (get 'a) 'b)
 (assert (get 'd 'e) 'f)
-;(assert (get 'a 'b) 'c)
+(assert (get 'd 'g) 'h)
+(assert (get 'x 'y 'z) 'a)
+;(operation-table 'display-proc) (newline)
 
+(display "\nex-3.26 - table via binary tree\n")
+(display "[see comments]\n")
 
-(display "\nex-3.26\n")
+; For reads and writes in O(log n) we can use a binary tree in each level of
+; the table. We would then implement assoc as a binary search. Inserts would be
+; handle as an insert into the binary tree.
+
+(display "\nex-3.27 - memoized fib\n")
+(display "[see comments]\n")
+
+; Explain why memo-fib computes the nth Fibonacci number in a number of steps
+; proportional to n? The procedures computes the result for each n only once.
+; If the result has already been calculated it is looked up in O(1). Hence, the
+; runtime is O(n).
+
+; Would the scheme still work if we had simply defined memo-fib to be (memoize
+; fib)? No, because only value for the initial call would be memoized. All
+; other values, would still be computed recursively without being memoized.
 
diff --git a/ex-3_28-xx.scm b/ex-3_28-xx.scm
new file mode 100644
index 0000000..92ec8f2
--- /dev/null
+++ b/ex-3_28-xx.scm
@@ -0,0 +1,4 @@
+(load "util.scm")
+
+(display "\nex-3.28\n")
+