diff --git a/project_euler/problem_31/sol1.py b/project_euler/problem_31/sol1.py
index 09b60cdae89d..fecf3e86958f 100644
--- a/project_euler/problem_31/sol1.py
+++ b/project_euler/problem_31/sol1.py
@@ -16,35 +16,35 @@ def one_pence():
     return 1
 
 
-def two_pence(x):
+def two_pence(x: int) -> int:
     return 0 if x < 0 else two_pence(x - 2) + one_pence()
 
 
-def five_pence(x):
+def five_pence(x: int) -> int:
     return 0 if x < 0 else five_pence(x - 5) + two_pence(x)
 
 
-def ten_pence(x):
+def ten_pence(x: int) -> int:
     return 0 if x < 0 else ten_pence(x - 10) + five_pence(x)
 
 
-def twenty_pence(x):
+def twenty_pence(x: int) -> int:
     return 0 if x < 0 else twenty_pence(x - 20) + ten_pence(x)
 
 
-def fifty_pence(x):
+def fifty_pence(x: int) -> int:
     return 0 if x < 0 else fifty_pence(x - 50) + twenty_pence(x)
 
 
-def one_pound(x):
+def one_pound(x: int) -> int:
     return 0 if x < 0 else one_pound(x - 100) + fifty_pence(x)
 
 
-def two_pound(x):
+def two_pound(x: int) -> int:
     return 0 if x < 0 else two_pound(x - 200) + one_pound(x)
 
 
-def solution(n):
+def solution(n: int) -> int:
     """Returns the number of different ways can n pence be made using any number of
     coins?