diff --git a/mtg.py b/mtg.py
index 104dc06..8caff24 100644
--- a/mtg.py
+++ b/mtg.py
@@ -1,6 +1,5 @@
 import copy
 import random
-import cards
 from observable import Observable
 
 class Game:
@@ -82,13 +81,15 @@ class ManaCost:
         elif isinstance(other, Mana):
             result.mana = other
         elif isinstance(other, ManaCost):
-            result.any = other.many
-            result.mana = other.many
+            result.any = self.any + other.any
+            result.mana = self.mana + other.mana
         return result
     def __sub__(self, other):
+        result = ManaCost()
         if isinstance(other, ManaCost):
-            result.any = self.any - right.any
-            result.mana = self.mana - right.mana
+            result.any = self.any - other.any
+            result.mana = self.mana - other.mana
+        return result
     def __repr__(self):
         return '{0}{1}'.format(
             self.any if self.any > 0 or self.mana.converted() == 0 else '',
diff --git a/test.py b/test.py
new file mode 100644
index 0000000..f57c65e
--- /dev/null
+++ b/test.py
@@ -0,0 +1,31 @@
+import unittest
+from mtg import *
+
+class TestManaMath(unittest.TestCase):
+    def test_add_mana(self):
+        self.assertEquals('RR', str(Mana('r') + Mana('r')))
+    def test_subtract_mana(self):
+        self.assertEquals('R', str(Mana('rr') - Mana('r')))
+    def test_subtract_mana_insufficient(self):
+        self.assertRaises(Exception, lambda: Mana('r') - Mana('rr'))
+        self.assertRaises(Exception, lambda: Mana('r') - Mana('g'))
+
+class TestManaCostMath(unittest.TestCase):
+    def test_add_manacost(self):
+        self.assertEquals('3R', str(ManaCost('2R') + ManaCost('1')))
+    def test_subtract_manacost(self):
+        self.assertEquals('1G', str(ManaCost('4GR') - ManaCost('3R')))
+    def test_subtract_manacost_from_mana(self):
+        result = Mana('UUBWWR') - ManaCost('1RW')
+        # Can't check the exact string, since there's no way of knowing which
+        # colors will be used for the additional mana requirement
+        self.assertEquals(3, result.converted())
+        self.assertEquals(0, result.mana['R'])
+        self.assertEquals(1, result.mana['W'])
+    def test_subtract_manacost_from_mana_insufficient_addtl(self):
+        self.assertRaises(Exception, lambda: Mana('RRW'), ManaCost('3R'))
+    def test_subtract_manacost_from_mana_insufficient_color(self):
+        self.assertRaises(Exception, lambda: Mana('RRW'), ManaCost('2U'))
+
+if __name__ == '__main__':
+    unittest.main()
\ No newline at end of file