diff --git a/practice/prep3.py b/practice/prep3.py
new file mode 100755
index 0000000..d249762
--- /dev/null
+++ b/practice/prep3.py
@@ -0,0 +1,133 @@
+"""CSC110 Fall 2020 Prep 3: Programming Exercises
+
+Instructions (READ THIS FIRST!)
+===============================
+
+This Python module contains several function headers and descriptions.
+Your task is to complete this module by doing the following for EACH function below:
+
+1. Write precondition expressions in each function docstring, based on the English
+   descriptions given. Each precondition expression must be valid Python code,
+   and preceded by a "- " (see format in Course Notes Section 3.7).
+2. Implement the function (i.e., write the function body so that the function
+   does what the description claims).
+
+You do NOT need to add additional doctests.
+
+We have marked each place you need to write preconditions/code with the word "TODO".
+As you complete your work in this file, delete each TODO comment---this is a
+good habit to get into early!
+
+At the bottom of this file, we've included code in the "main" block for running
+doctest example and running python_ta.check_all to check your submission for
+this prep. But we've ALSO added the python_ta.contracts part (described in 3.7)
+to help you check your precondition expressions. To check a precondition expression:
+
+1. Write the expression under "Preconditions:" in the docstring.
+2. Run this file in the Python console. (Right-click -> Run File in Python Console)
+3. Try calling the function with arguments that VIOLATE (make False) your precondition.
+4. You should see an "AssertionError", indicating that python_ta checked your
+   precondition expression and stopped the function call.
+
+
+Copyright and Usage Information
+===============================
+
+This file is provided solely for the personal and private use of students
+taking CSC110 at the University of Toronto St. George campus. All forms of
+distribution of this code, whether as given or with any changes, are
+expressly prohibited. For more information on copyright for CSC110 materials,
+please consult our Course Syllabus.
+
+This file is Copyright (c) 2020 David Liu and Mario Badr.
+"""
+
+
+def same_at_index(s1: str, s2: str, index: int) -> bool:
+    """Return whether s1 and s2 have the same character at the given index.
+
+    This assumes that index is >= 0 and a valid index for both s1 and s2.
+
+    Preconditions:
+      - index >= 0
+      - len(s1) > index
+      - len(s2) > index
+
+    >>> same_at_index('Mario', 'David', 1)
+    True
+    >>> same_at_index('Hi', 'Bye', 0)
+    False
+    """
+    return s1[index] == s2[index]
+
+
+def bigger_max(nums1: set, nums2: set) -> set:
+    """Return the set that has the larger maximum.
+
+    Return nums1 if there is a tie.
+
+    This assumes that both sets are non-empty, and that they only contain integers.
+
+    NOTE: Use the builtin function isinstance to check whether a value has a certain
+    type. For example, isinstance(3, int) is True, and isinstance('hi', int) is False.
+
+    Preconditions:
+      - len(nums1) > 0
+      - len(nums2) > 0
+      - all([isinstance(val, int) for val in nums1])
+      - all([isinstance(val, int) for val in nums2])
+
+    >>> bigger_max({1, 2, 3}, {4})
+    {4}
+    >>> bigger_max({1, 2, 3}, {1, 3})
+    {1, 2, 3}
+    >>> bigger_max({1, 3}, {1, 2, 3})
+    {1, 3}
+    """
+    max1 = max(nums1)
+    max2 = max(nums2)
+    big_max = max(max1, max2)
+    # This always puts overrides max2 with max1 when they have the same max
+    return {max2: nums2, max1: nums1}[big_max]
+
+
+def lookup_with_backup(mapping: dict, key: object, backup_key: object) -> object:
+    """Return the corresponding value of key in mapping.
+
+    If key is not in mapping, then return the corresponding value of of backup_key
+    in mapping instead.
+
+    This assumes that at least one of key and backup_key are a key in map.
+
+    NOTE: the type contract here uses "object" for key, backup_key, and the return type.
+    We've included this so that you do *not* need to write any preconditions to check
+    for the type of the keys or corresponding values in map.
+
+    Preconditions:
+      - key in mapping or backup_key in mapping
+
+    >>> example_dict = {'Burger': 5.0, 'Fries': 3.0}
+    >>> lookup_with_backup(example_dict, 'Fries', 'Burger')
+    3.0
+    >>> lookup_with_backup(example_dict, 'Cheeseburger', 'Burger')
+    5.0
+    """
+    return mapping[key] if key in mapping else mapping[backup_key]
+
+
+if __name__ == '__main__':
+    import python_ta.contracts
+    python_ta.contracts.DEBUG_CONTRACTS = False
+    python_ta.contracts.check_all_contracts()
+
+    import doctest
+    doctest.testmod(verbose=True)
+
+    # When you are ready, uncomment the following lines to check
+    # your work with PythonTA.
+    import python_ta
+    python_ta.check_all(config={
+        'extra-imports': ['python_ta.contracts'],
+        'max-line-length': 100,
+        'disable': ['R1705']
+    })