diff --git a/solutions/system_design/mint/mint_mapreduce.py b/solutions/system_design/mint/mint_mapreduce.py
index 2e8339f..3dc8855 100644
--- a/solutions/system_design/mint/mint_mapreduce.py
+++ b/solutions/system_design/mint/mint_mapreduce.py
@@ -35,7 +35,7 @@ class SpendingByCategory(MRJob):
         if period == self.current_year_month():
             yield (period, category), amount
 
-    def reducer(self, key, value):
+    def reducer(self, key, values):
         """Sum values for each key.
 
         (2016-01, shopping), 125
diff --git a/solutions/system_design/pastebin/pastebin.py b/solutions/system_design/pastebin/pastebin.py
index 7cb1f20..7e8d268 100644
--- a/solutions/system_design/pastebin/pastebin.py
+++ b/solutions/system_design/pastebin/pastebin.py
@@ -26,7 +26,7 @@ class HitCounts(MRJob):
         period = self.extract_year_month(line)
         yield (period, url), 1
 
-    def reducer(self, key, value):
+    def reducer(self, key, values):
         """Sum values for each key.
 
         (2016-01, url0), 2
diff --git a/solutions/system_design/sales_rank/sales_rank_mapreduce.py b/solutions/system_design/sales_rank/sales_rank_mapreduce.py
index bbe844b..6eeeb52 100644
--- a/solutions/system_design/sales_rank/sales_rank_mapreduce.py
+++ b/solutions/system_design/sales_rank/sales_rank_mapreduce.py
@@ -9,7 +9,7 @@ class SalesRanker(MRJob):
         """Return True if timestamp is within past week, False otherwise."""
         ...
 
-    def mapper(self, _ line):
+    def mapper(self, _, line):
         """Parse each log line, extract and transform relevant lines.
 
         Emit key value pairs of the form:
@@ -25,7 +25,7 @@ class SalesRanker(MRJob):
         if self.within_past_week(timestamp):
             yield (category, product_id), quantity
 
-    def reducer(self, key, value):
+    def reducer(self, key, values):
         """Sum values for each key.
 
         (foo, p1), 2
@@ -74,4 +74,4 @@ class SalesRanker(MRJob):
 
 
 if __name__ == '__main__':
-    HitCounts.run()
+    SalesRanker.run()