diff --git a/README.md b/README.md
index e49fc04..acd4530 100644
--- a/README.md
+++ b/README.md
@@ -11,9 +11,14 @@ if your manager determines the performance is too lacking, you will be asked to
 Please include the following when you are writing your PR:   
 General things:   
 1. What is the purpose of this PR?
+complete last level
 2. What changes did you make? Why?
+compiled the system.
 
 This PR Specific:
 1. How did you design your system?
+basically used dict to store and retrive data. seems very basic
 2. What did you struggle with?
+n/a
 3. Is there anything you would change about this step?
+instructios seem vague?
diff --git a/src/interface.py b/src/interface.py
index 65d331e..7c0c513 100644
--- a/src/interface.py
+++ b/src/interface.py
@@ -1,13 +1,36 @@
-class Exchange:
-    # implement this!
+class TradeObject:
 
-    def __init__(self, initialBalance):
-        """Initial Balance is the amount that each account should start with."""
-        pass
+    def __init__(self, seller, buyer, ticker, price, amount):
+        self.seller = seller #str
+        self.buyer = buyer #str
+        self.ticker = ticker #str
+        self.price = price #price > 0
+        self.amount = amount #amount > 0
 
-    def add_trade(self, trade):
-        """Adds a trade to the exchange (validation required)
-        and returns a match if required. It is up to you on how you will
-        handle representing trades. """
-        raise NotImplementedError
+class ExchangeUser:
 
+    def __init__(self, usrName = 'testname'):
+        self.holdings = {} # ticker: (amount, avgPrice)
+        self.name = usrName
+
+    def add_trade(self, trade:TradeObject):
+        ticker = trade.ticker
+        if self.name == trade.buyer:
+            if ticker in self.holdings:
+                #calculates the avg cost after buying at different prices.
+                self.holdings[ticker] = (self.holdings[ticker][0] + trade.amount, (self.holdings[ticker][1] * self.holdings[ticker][0] + trade.price * trade.amount) / (self.holdings[ticker][0] + trade.amount))
+                if self.holdings[ticker][0] == 0:
+                    del self.holdings[ticker]
+            else:
+                self.holdings[ticker] = (trade.amount, trade.price)
+
+        elif self.name == trade.seller:
+            if ticker in self.holdings:
+                self.holdings[ticker] = (self.holdings[ticker][0] - trade.amount, self.holdings[ticker][1])
+                if self.holdings[ticker][0] == 0:
+                    del self.holdings[ticker]
+            else:
+                self.holdings[ticker] = (-trade.amount, trade.price)
+
+    def getHoldings(self):
+        return self.holdings
\ No newline at end of file