diff --git a/financial/ABOUT.md b/financial/ABOUT.md
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+### Interest
+
+* Compound Interest: "Compound interest is calculated by multiplying the initial principal amount by one plus the annual interest rate raised to the number of compound periods minus one." [Compound Interest](https://www.investopedia.com/)
+* Simple Interest: "Simple interest paid or received over a certain period is a fixed percentage of the principal amount that was borrowed or lent. " [Simple Interest](https://www.investopedia.com/)
diff --git a/financial/__init__.py b/financial/__init__.py
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diff --git a/financial/interest.py b/financial/interest.py
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+# https://www.investopedia.com
+
+from __future__ import annotations
+
+
+def simple_interest(
+    principle: float, daily_interest_rate: float, days_between_payments: int
+) -> float:
+    """
+    >>> simple_interest(18000.0, 0.06, 3)
+    3240.0
+    >>> simple_interest(0.5, 0.06, 3)
+    0.09
+    >>> simple_interest(18000.0, 0.01, 10)
+    1800.0
+    >>> simple_interest(18000.0, 0.0, 3)
+    0.0
+    >>> simple_interest(5500.0, 0.01, 100)
+    5500.0
+    >>> simple_interest(10000.0, -0.06, 3)
+    Traceback (most recent call last):
+        ...
+    ValueError: daily_interest_rate must be >= 0
+    >>> simple_interest(-10000.0, 0.06, 3)
+    Traceback (most recent call last):
+        ...
+    ValueError: principle must be > 0
+    >>> simple_interest(5500.0, 0.01, -5)
+    Traceback (most recent call last):
+        ...
+    ValueError: days_between_payments must be > 0
+    """
+    if days_between_payments <= 0:
+        raise ValueError("days_between_payments must be > 0")
+    if daily_interest_rate < 0:
+        raise ValueError("daily_interest_rate must be >= 0")
+    if principle <= 0:
+        raise ValueError("principle must be > 0")
+    return principle * daily_interest_rate * days_between_payments
+
+
+def compound_interest(
+    principle: float,
+    nominal_annual_interest_rate_percentage: float,
+    number_of_compounding_periods: int,
+) -> float:
+    """
+    >>> compound_interest(10000.0, 0.05, 3)
+    1576.2500000000014
+    >>> compound_interest(10000.0, 0.05, 1)
+    500.00000000000045
+    >>> compound_interest(0.5, 0.05, 3)
+    0.07881250000000006
+    >>> compound_interest(10000.0, 0.06, -4)
+    Traceback (most recent call last):
+        ...
+    ValueError: number_of_compounding_periods must be > 0
+    >>> compound_interest(10000.0, -3.5, 3.0)
+    Traceback (most recent call last):
+        ...
+    ValueError: nominal_annual_interest_rate_percentage must be >= 0
+    >>> compound_interest(-5500.0, 0.01, 5)
+    Traceback (most recent call last):
+        ...
+    ValueError: principle must be > 0
+    """
+    if number_of_compounding_periods <= 0:
+        raise ValueError("number_of_compounding_periods must be > 0")
+    if nominal_annual_interest_rate_percentage < 0:
+        raise ValueError("nominal_annual_interest_rate_percentage must be >= 0")
+    if principle <= 0:
+        raise ValueError("principle must be > 0")
+
+    return principle * (
+        (1 + nominal_annual_interest_rate_percentage) ** number_of_compounding_periods
+        - 1
+    )
+
+
+if __name__ == "__main__":
+    import doctest
+
+    doctest.testmod()