diff --git a/project_euler/problem_01/sol1.py b/project_euler/problem_01/sol1.py
index e81156eda..2dbb60cdb 100644
--- a/project_euler/problem_01/sol1.py
+++ b/project_euler/problem_01/sol1.py
@@ -6,7 +6,7 @@ Find the sum of all the multiples of 3 or 5 below N.
 """
 
 
-def solution(n):
+def solution(n: int = 1000) -> int:
     """Returns the sum of all the multiples of 3 or 5 below n.
 
     >>> solution(3)
diff --git a/project_euler/problem_01/sol2.py b/project_euler/problem_01/sol2.py
index 8041c7ffa..212fd4056 100644
--- a/project_euler/problem_01/sol2.py
+++ b/project_euler/problem_01/sol2.py
@@ -6,7 +6,7 @@ Find the sum of all the multiples of 3 or 5 below N.
 """
 
 
-def solution(n):
+def solution(n: int = 1000) -> int:
     """Returns the sum of all the multiples of 3 or 5 below n.
 
     >>> solution(3)
diff --git a/project_euler/problem_01/sol3.py b/project_euler/problem_01/sol3.py
index c0bcbc06e..faa505615 100644
--- a/project_euler/problem_01/sol3.py
+++ b/project_euler/problem_01/sol3.py
@@ -6,7 +6,7 @@ Find the sum of all the multiples of 3 or 5 below N.
 """
 
 
-def solution(n):
+def solution(n: int = 1000) -> int:
     """
     This solution is based on the pattern that the successive numbers in the
     series follow: 0+3,+2,+1,+3,+1,+2,+3.
diff --git a/project_euler/problem_01/sol4.py b/project_euler/problem_01/sol4.py
index e01dc977d..d5e86320d 100644
--- a/project_euler/problem_01/sol4.py
+++ b/project_euler/problem_01/sol4.py
@@ -6,7 +6,7 @@ Find the sum of all the multiples of 3 or 5 below N.
 """
 
 
-def solution(n):
+def solution(n: int = 1000) -> int:
     """Returns the sum of all the multiples of 3 or 5 below n.
 
     >>> solution(3)
diff --git a/project_euler/problem_01/sol5.py b/project_euler/problem_01/sol5.py
index bd96d965f..eae62ef5c 100644
--- a/project_euler/problem_01/sol5.py
+++ b/project_euler/problem_01/sol5.py
@@ -8,7 +8,7 @@ Find the sum of all the multiples of 3 or 5 below N.
 """A straightforward pythonic solution using list comprehension"""
 
 
-def solution(n):
+def solution(n: int = 1000) -> int:
     """Returns the sum of all the multiples of 3 or 5 below n.
 
     >>> solution(3)
diff --git a/project_euler/problem_01/sol6.py b/project_euler/problem_01/sol6.py
index c9f94b9f7..ca08a4a63 100644
--- a/project_euler/problem_01/sol6.py
+++ b/project_euler/problem_01/sol6.py
@@ -6,7 +6,7 @@ Find the sum of all the multiples of 3 or 5 below N.
 """
 
 
-def solution(n):
+def solution(n: int = 1000) -> int:
     """Returns the sum of all the multiples of 3 or 5 below n.
 
     >>> solution(3)
diff --git a/project_euler/problem_01/sol7.py b/project_euler/problem_01/sol7.py
index a0510b54c..8e53d2a81 100644
--- a/project_euler/problem_01/sol7.py
+++ b/project_euler/problem_01/sol7.py
@@ -6,7 +6,7 @@ Find the sum of all the multiples of 3 or 5 below N.
 """
 
 
-def solution(n):
+def solution(n: int = 1000) -> int:
     """Returns the sum of all the multiples of 3 or 5 below n.
 
     >>> solution(3)