diff --git a/ProjectEulerCS/ProblemSelection.cs b/ProjectEulerCS/ProblemSelection.cs
index 88fef99..dc722f8 100644
--- a/ProjectEulerCS/ProblemSelection.cs
+++ b/ProjectEulerCS/ProblemSelection.cs
@@ -32,7 +32,7 @@ namespace ProjectEulerCS{
 		private static readonly List<int> _PROBLEM_NUMBERS = new List<int>()
 				{ 0,  1,  2,  3,  4,  5,  6,  7,  8,  9,
 				 10, 11, 12, 13, 14, 15, 16, 17, 18, 19,
-				 20, 67};
+				 20, 21, 67};
 		public static System.Collections.Generic.List<int> PROBLEM_NUMBERS{
 			get { return _PROBLEM_NUMBERS; }
 		}
@@ -61,6 +61,7 @@ namespace ProjectEulerCS{
 				case 18: problem = new Problem18(); break;
 				case 19: problem = new Problem19(); break;
 				case 20: problem = new Problem20(); break;
+				case 21: problem = new Problem21(); break;
 				case 67: problem = new Problem67(); break;
 			}
 			return problem;
diff --git a/ProjectEulerCS/Problems/Problem20.cs b/ProjectEulerCS/Problems/Problem20.cs
index 59b148a..70d310e 100644
--- a/ProjectEulerCS/Problems/Problem20.cs
+++ b/ProjectEulerCS/Problems/Problem20.cs
@@ -1,4 +1,4 @@
-//ProjectEuler/ProjectEulerCS/src/Problems/Problem19.cs
+//ProjectEuler/ProjectEulerCS/src/Problems/Problem20.cs
 //Matthew Ellison
 // Created: 08-27-20
 //Modified: 08-27-20
diff --git a/ProjectEulerCS/Problems/Problem21.cs b/ProjectEulerCS/Problems/Problem21.cs
new file mode 100644
index 0000000..e9c8213
--- /dev/null
+++ b/ProjectEulerCS/Problems/Problem21.cs
@@ -0,0 +1,151 @@
+//ProjectEuler/ProjectEulerCS/src/Problems/Problem21.cs
+//Matthew Ellison
+// Created: 09-02-20
+//Modified: 09-02-20
+//Evaluate the sum of all the amicable numbers under 10000
+//Unless otherwise listed all non-standard includes are my own creation and available from https://bibucket.org/Mattrixwv/CSClasses
+/*
+	Copyright (C) 2020  Matthew Ellison
+
+	This program is free software: you can redistribute it and/or modify
+	it under the terms of the GNU Lesser General Public License as published by
+	the Free Software Foundation, either version 3 of the License, or
+	(at your option) any later version.
+
+	This program is distributed in the hope that it will be useful,
+	but WITHOUT ANY WARRANTY; without even the implied warranty of
+	MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+	GNU Lesser General Public License for more details.
+
+	You should have received a copy of the GNU Lesser General Public License
+	along with this program.  If not, see <https://www.gnu.org/licenses/>.
+*/
+
+
+using System.Collections.Generic;
+
+
+namespace ProjectEulerCS.Problems{
+	public class Problem21 : Problem{
+		//Variables
+		//Static variables
+		private const int LIMIT = 10000;	//The number that is > the largest number to be checked
+		//Instance variables
+		private List<int> divisorSum;	//Holds the sum of the divisors of the subscript number
+		private List<int> amicable;		//Holds all amicable numbers
+		public List<int> Amicable{
+			get{
+				if(!solved){
+					throw new Unsolved();
+				}
+				return amicable;
+			}
+		}
+		public int Sum{
+			get{
+				if(!solved){
+					throw new Unsolved();
+				}
+				return mee.Algorithms.GetSum(amicable);
+			}
+		}
+		public override string Result{
+			get{
+				if(!solved){
+					throw new Unsolved();
+				}
+				string result = $"All amicable numebrs less than {LIMIT} are\n";
+				foreach (int num in amicable){
+					result += $"{num}\n";
+				}
+				result += $"The sum of all of these amicable numbers is {Sum}";
+				return result;
+			}
+		}
+
+		//Functions
+		//Constructor
+		public Problem21() : base($"Evaluate the sum of all the amicable numbers under {LIMIT}"){
+			divisorSum = new List<int>();
+			amicable = new List<int>();
+			reserveArray();
+		}
+		//Operational functions
+		//Reserve the size of the array to speed up insertion
+		private void reserveArray(){
+			divisorSum.Capacity = LIMIT;	//Reserving it now makes it faster later
+			//Make sure the list is filled with 0's
+			while(divisorSum.Count < LIMIT){
+				divisorSum.Add(0);
+			}
+		}
+		//Solve the problem
+		public override void Solve(){
+			//If the problem has already been solved do nothing and end the function
+			if(solved){
+				return;
+			}
+
+			//Start the timer
+			timer.Start();
+
+			//Generate the divisors of all numbers < 10000, get their sum, and add it to the list
+			for(int cnt = 1;cnt < LIMIT;++cnt){
+				List<int> divisors = mee.Algorithms.GetDivisors(cnt);	//Get all the divisors of a number
+				if(divisors.Count > 1){
+					divisors.Remove(divisors[^1]);	//Remove the last entry because it will be the number itself
+				}
+				divisorSum[cnt] = mee.Algorithms.GetSum(divisors);	//Add the sum of the divisors of the vector
+			}
+			//Check every sum of divisors in the list for a matching sum
+			for(int cnt = 1;cnt < divisorSum.Count;++cnt){
+				int sum = divisorSum[cnt];
+				//If the sum is greater than the number of divisors then it is impossible to be amicable. Skip the number and continue
+				if(sum >= divisorSum.Count){
+					continue;
+				}
+				//We know that divisorSum.at(cnt) == sum, so if divisorSum.at(sum) == cnt we found an amicable number
+				if(divisorSum[sum] == cnt){
+					//A number can't be amicable with itself
+					if(sum == cnt){
+						continue;
+					}
+					//Add it to the list of amicable numbers
+					amicable.Add(cnt);
+				}
+			}
+
+			//Sort the list for neatness
+			amicable.Sort();
+
+			//Stop the timer
+			timer.Stop();
+
+			//Throw a flag to show the problem is solved
+			solved = true;
+		}
+		//Reset the problem so it can be run again
+		public override void Reset(){
+			base.Reset();
+			divisorSum.Clear();
+			amicable.Clear();
+			reserveArray();
+		}
+	}
+}
+
+/* Results:
+All amicable numebrs less than 10000 are
+220
+284
+1184
+1210
+2620
+2924
+5020
+5564
+6232
+6368
+The sum of all of these amicable numbers is 31626
+It took an average of 5.076 milliseconds to run this problem through 100 iterations
+*/