diff --git a/lib/problems.rb b/lib/problems.rb
index 5085953d..fea1a11e 100644
--- a/lib/problems.rb
+++ b/lib/problems.rb
@@ -1,13 +1,52 @@
 require_relative './stack.rb'
 
-# Time Complexity: ?
-# Space Complexity: ?
+# Time Complexity: O(n) - goes through all input elements
+# Space Complexity: O(n) - stack will take at least half of symbols in string
 def balanced(string)
-  raise NotImplementedError, "Not implemented yet"
+  return true if string.length == 0
+
+  pairs = {"(" => ")", "{" => "}", "[" => "]"}
+  expectations = Stack.new
+  string.chars.each do |char|
+    if pairs[char]
+      expected_element = pairs[char]
+      expectations.push(expected_element)
+    else
+      if expectations.pop != char
+        return false
+      end
+    end
+  end
+  return expectations.empty?
 end
 
-# Time Complexity: ?
-# Space Complexity: ?
+# Time Complexity: O(n) - goes through all input elements
+# Space Complexity: O(1) - stack will never have more than 2 elements
 def evaluate_postfix(postfix_expression)
-  raise NotImplementedError, "Not implemented yet"
-end
+  operands = Stack.new
+  postfix_expression.chars.each do |element|
+    if element.to_i.to_s == element
+      operands.push(element.to_i)
+    else
+      num2 = operands.pop
+      num1 = operands.pop
+      case element
+      when "+"   
+        answer = num1 + num2        
+      when "*"       
+        answer = num1 * num2    
+      when "-"        
+        answer = num1 - num2     
+      when "/"        
+        answer = num1 / num2    
+      end
+      operands.push(answer)
+    end
+  end
+  result = operands.pop
+  if operands.empty?
+    return result
+  else
+    raise Exception.new "Expression is not valid"
+  end
+end
\ No newline at end of file
diff --git a/lib/queue.rb b/lib/queue.rb
index 828217c6..5633cc38 100644
--- a/lib/queue.rb
+++ b/lib/queue.rb
@@ -1,31 +1,54 @@
 class Queue
 
   def initialize
-    # @store = ...
-    raise NotImplementedError, "Not yet implemented"
+    @store = Array.new
+    @size = 20
+    @start = 0
+    @end = 0 
   end
 
   def enqueue(element)
-    raise NotImplementedError, "Not yet implemented"
+    # check if we can add more elements
+    raise Exception.new "No vacant spots in queue" if @size < self.size
+
+    @store[@end] = element
+    @end = (@end + 1) % @size
   end
 
   def dequeue
-    raise NotImplementedError, "Not yet implemented"
+    raise Exception.new "No elements in queue" if self.size == 0
+    element = @store[@start]
+    @store[@start] = nil
+    @start = (@start + 1) % @size
+    return element
   end
 
   def front
-    raise NotImplementedError, "Not yet implemented"
+    raise Exception.new "No elements in queue" if self.size == 0
+    @store[@start]
   end
 
   def size
-    raise NotImplementedError, "Not yet implemented"
+    if @end == @start && @store[@start]
+      return 20
+    elsif @end > @start
+      return @end - @start
+    elsif @end < @start
+      return @end + @size - @start
+    else
+      return 0
+    end
   end
 
   def empty?
-    raise NotImplementedError, "Not yet implemented"
+    @end == @start
   end
 
   def to_s
-    return @store.to_s
+    if @end > @start
+      return @store[@start..@end].to_s
+    else
+      return (@store[@start..@size-1] + @store[0..@end-1]).to_s
+    end
   end
 end
diff --git a/lib/stack.rb b/lib/stack.rb
index cfc6ef0f..966e0492 100644
--- a/lib/stack.rb
+++ b/lib/stack.rb
@@ -1,19 +1,20 @@
+require_relative './linked_list.rb'
+
 class Stack
   def initialize
-    # @store = ...
-    raise NotImplementedError, "Not yet implemented"
+    @store = LinkedList.new
   end
 
   def push(element)
-    raise NotImplementedError, "Not yet implemented"
+    @store.add_first(element)
   end
 
   def pop
-    raise NotImplementedError, "Not yet implemented"
+    @store.remove_first()
   end
 
   def empty?
-    raise NotImplementedError, "Not yet implemented"
+    @store.empty?
   end
 
   def to_s
diff --git a/test/problems_test.rb b/test/problems_test.rb
index 046f059e..c0cfc618 100644
--- a/test/problems_test.rb
+++ b/test/problems_test.rb
@@ -2,7 +2,7 @@
 
 Minitest::Reporters.use! Minitest::Reporters::SpecReporter.new
 
-xdescribe "Test wave 3 problems" do
+describe "Test wave 3 problems" do
   describe "balanced" do
     it "Given balanced strings it should return true" do
       
diff --git a/test/test_helper.rb b/test/test_helper.rb
index 426168c8..f9340080 100644
--- a/test/test_helper.rb
+++ b/test/test_helper.rb
@@ -10,4 +10,4 @@
 require_relative "../lib/queue.rb"
 require_relative "../lib/stack.rb"
 # Extra exercises
-# require_relative "../lib/problems.rb"
\ No newline at end of file
+require_relative "../lib/problems.rb"
\ No newline at end of file