diff --git a/project_euler/problem_01/sol1.py b/project_euler/problem_01/sol1.py
index e81156edaee4..ff62bd6ef324 100644
--- a/project_euler/problem_01/sol1.py
+++ b/project_euler/problem_01/sol1.py
@@ -1,12 +1,13 @@
+# noqa: flake
 """
 Problem Statement:
 If we list all the natural numbers below 10 that are multiples of 3 or 5,
-we get 3,5,6 and 9. The sum of these multiples is 23.
+we get 3, 5, 6 and 9. The sum of these multiples is 23.
 Find the sum of all the multiples of 3 or 5 below N.
 """
 
 
-def solution(n):
+def solution(n: int) -> int:
     """Returns the sum of all the multiples of 3 or 5 below n.
 
     >>> solution(3)
@@ -21,7 +22,7 @@ def solution(n):
     0
     """
 
-    return sum([e for e in range(3, n) if e % 3 == 0 or e % 5 == 0])
+    return sum(e for e in range(3, n) if e % 3 == 0 or e % 5 == 0)
 
 
 if __name__ == "__main__":
diff --git a/project_euler/problem_01/sol2.py b/project_euler/problem_01/sol2.py
index 8041c7ffa589..022c1e0a1957 100644
--- a/project_euler/problem_01/sol2.py
+++ b/project_euler/problem_01/sol2.py
@@ -1,12 +1,12 @@
 """
 Problem Statement:
 If we list all the natural numbers below 10 that are multiples of 3 or 5,
-we get 3,5,6 and 9. The sum of these multiples is 23.
+we get 3, 5, 6 and 9. The sum of these multiples is 23.
 Find the sum of all the multiples of 3 or 5 below N.
 """
 
 
-def solution(n):
+def solution(n: int) -> int:
     """Returns the sum of all the multiples of 3 or 5 below n.
 
     >>> solution(3)
@@ -19,14 +19,14 @@ def solution(n):
     83700
     """
 
-    sum = 0
+    total = 0
     terms = (n - 1) // 3
-    sum += ((terms) * (6 + (terms - 1) * 3)) // 2  # sum of an A.P.
+    total += ((terms) * (6 + (terms - 1) * 3)) // 2  # total of an A.P.
     terms = (n - 1) // 5
-    sum += ((terms) * (10 + (terms - 1) * 5)) // 2
+    total += ((terms) * (10 + (terms - 1) * 5)) // 2
     terms = (n - 1) // 15
-    sum -= ((terms) * (30 + (terms - 1) * 15)) // 2
-    return sum
+    total -= ((terms) * (30 + (terms - 1) * 15)) // 2
+    return total
 
 
 if __name__ == "__main__":
diff --git a/project_euler/problem_01/sol3.py b/project_euler/problem_01/sol3.py
index c0bcbc06ec83..b4c5ca72c4d0 100644
--- a/project_euler/problem_01/sol3.py
+++ b/project_euler/problem_01/sol3.py
@@ -1,12 +1,12 @@
 """
 Problem Statement:
 If we list all the natural numbers below 10 that are multiples of 3 or 5,
-we get 3,5,6 and 9. The sum of these multiples is 23.
+we get 3, 5, 6 and 9. The sum of these multiples is 23.
 Find the sum of all the multiples of 3 or 5 below N.
 """
 
 
-def solution(n):
+def solution(n: int) -> int:
     """
     This solution is based on the pattern that the successive numbers in the
     series follow: 0+3,+2,+1,+3,+1,+2,+3.
@@ -22,38 +22,38 @@ def solution(n):
     83700
     """
 
-    sum = 0
+    total = 0
     num = 0
     while 1:
         num += 3
         if num >= n:
             break
-        sum += num
+        total += num
         num += 2
         if num >= n:
             break
-        sum += num
+        total += num
         num += 1
         if num >= n:
             break
-        sum += num
+        total += num
         num += 3
         if num >= n:
             break
-        sum += num
+        total += num
         num += 1
         if num >= n:
             break
-        sum += num
+        total += num
         num += 2
         if num >= n:
             break
-        sum += num
+        total += num
         num += 3
         if num >= n:
             break
-        sum += num
-    return sum
+        total += num
+    return total
 
 
 if __name__ == "__main__":
diff --git a/project_euler/problem_01/sol4.py b/project_euler/problem_01/sol4.py
index e01dc977d8cf..149a19d505c3 100644
--- a/project_euler/problem_01/sol4.py
+++ b/project_euler/problem_01/sol4.py
@@ -1,12 +1,12 @@
 """
 Problem Statement:
 If we list all the natural numbers below 10 that are multiples of 3 or 5,
-we get 3,5,6 and 9. The sum of these multiples is 23.
+we get 3, 5, 6 and 9. The sum of these multiples is 23.
 Find the sum of all the multiples of 3 or 5 below N.
 """
 
 
-def solution(n):
+def solution(n: int) -> 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 bd96d965f92d..31926a2dc2c9 100644
--- a/project_euler/problem_01/sol5.py
+++ b/project_euler/problem_01/sol5.py
@@ -1,15 +1,14 @@
 """
 Problem Statement:
 If we list all the natural numbers below 10 that are multiples of 3 or 5,
-we get 3,5,6 and 9. The sum of these multiples is 23.
+we get 3, 5, 6 and 9. The sum of these multiples is 23.
 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) -> int:
     """Returns the sum of all the multiples of 3 or 5 below n.
+       A straightforward pythonic solution using comprehension.
 
     >>> solution(3)
     0
@@ -21,7 +20,7 @@ def solution(n):
     83700
     """
 
-    return sum([i for i in range(n) if i % 3 == 0 or i % 5 == 0])
+    return sum(i for i in range(n) if i % 3 == 0 or i % 5 == 0)
 
 
 if __name__ == "__main__":
diff --git a/project_euler/problem_01/sol6.py b/project_euler/problem_01/sol6.py
index c9f94b9f77c8..31f8bc9b6e8f 100644
--- a/project_euler/problem_01/sol6.py
+++ b/project_euler/problem_01/sol6.py
@@ -1,12 +1,12 @@
 """
 Problem Statement:
 If we list all the natural numbers below 10 that are multiples of 3 or 5,
-we get 3,5,6 and 9. The sum of these multiples is 23.
+we get 3, 5, 6 and 9. The sum of these multiples is 23.
 Find the sum of all the multiples of 3 or 5 below N.
 """
 
 
-def solution(n):
+def solution(n: int) -> 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 a0510b54c409..593fed299aa9 100644
--- a/project_euler/problem_01/sol7.py
+++ b/project_euler/problem_01/sol7.py
@@ -1,12 +1,12 @@
 """
 Problem Statement:
 If we list all the natural numbers below 10 that are multiples of 3 or 5,
-we get 3,5,6 and 9. The sum of these multiples is 23.
+we get 3, 5, 6 and 9. The sum of these multiples is 23.
 Find the sum of all the multiples of 3 or 5 below N.
 """
 
 
-def solution(n):
+def solution(n: int) -> int:
     """Returns the sum of all the multiples of 3 or 5 below n.
 
     >>> solution(3)