diff --git a/15_postfix.lua b/15_postfix.lua
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+-- Postfix notation is an alternative to the normal "infix" notation
+-- used in mathematical expressions.  Instead of writing the operator
+-- between the operands, the operator comes after them.  For example,
+-- "1 + 2" is notated as "1 2 +".  This removes the need for operator
+-- precedence and parentheses: rather than having to resolve the
+-- ambiguity of an expression like "1 + 2 * 3" with convention or
+-- parentheses, the evaluation order is explicit -- multiplication-
+-- first is "1 2 3 * +" and addition-first is "1 2 + 3 *".
+--
+-- As well as this notational convenience, postfix can be easily
+-- evaluated using a stack.  Whenever an operand is encountered, the
+-- value is pushed onto the stack, and whenever an operator is
+-- encountered, two values are popped from the stack and the result of
+-- the operation is pushed on in their place.  Finally, the value left
+-- on the top of the stack is the result of the operation.
+--
+-- Write a function that will evaluate a postfix expression and return
+-- the result.  You can assume that the expression is valid, and not
+-- worry about handling error cases.  The operators will be "+", "-",
+-- "*" and "/".
+--
+-- Solution --------------------------------------------------------------------
+function evaluate_postfix(expression)
+	-- Your implementation here
+end
+
+-- Tests -----------------------------------------------------------------------
+
+local luaunit = require("luaunit.luaunit")
+
+function test_1_2_plus_3_times_is_9()
+	luaunit.assertEquals(evaluate_postfix("1 2 + 3 *"), 9)
+end
+
+function test_1_2_3_times_plus_is_7()
+	luaunit.assertEquals(evaluate_postfix("1 2 3 * +"), 7)
+end
+
+function test_4_6_plus_5_divide_2_1_minus_minus_is_1()
+	luaunit.assertEquals(evaluate_postfix("4 6 + 5 / 2 1 - -"), 1)
+end
+
+function test_12_2_times_6_divide_10_times_2_plus_is_42()
+	luaunit.assertEquals(evaluate_postfix("12 2 * 6 / 10 * 2 +"), 42)
+end
+
+os.exit(luaunit.LuaUnit.run())