diff --git a/labs/11_hash_tables/README.md b/labs/11_hash_tables/README.md
index 53e336a..2c982c2 100644
--- a/labs/11_hash_tables/README.md
+++ b/labs/11_hash_tables/README.md
@@ -24,9 +24,38 @@ Finally, implement and test the *resize* function. This function is automaticall
 
 **To complete this checkpoint**: Show a TA these additions and the test output.
 
-## Checkpoint 3
+## Checkpoint 3: Using std::unordered_map
 
-*estimate: remainder of lab time*
+*estimate: 20-30 minutes*
 
-To be added.
+Complete the *isHappy* function in this [program](test_happy_number.cpp). This function determines if a number n is happy or not. You can assume 1<=n<50000. Do not include extra libraries.
 
+A happy number is a number defined by the following process:
+
+- Starting with any positive integer, replace the number by the sum of the squares of its digits.
+- Repeat the process until the number equals 1 (where it will stay), or it loops endlessly in a cycle which does not include 1.
+- Those numbers for which this process ends in 1 are happy.
+
+Return true if n is a happy number, and false if not. Here are some examples:
+
+```console
+Example 1:
+
+Input: n = 19
+Output: true
+
+Explanation:
+12 + 92 = 82
+82 + 22 = 68
+62 + 82 = 100
+12 + 02 + 02 = 1
+```
+
+```console
+Example 2:
+
+Input: n = 2
+Output: false
+```
+
+**To complete this checkpoint**: Show a TA your program and the test results.
diff --git a/labs/11_hash_tables/test_happy_number.cpp b/labs/11_hash_tables/test_happy_number.cpp
new file mode 100644
index 0000000..4b0b1a6
--- /dev/null
+++ b/labs/11_hash_tables/test_happy_number.cpp
@@ -0,0 +1,24 @@
+#include <iostream>
+#include <unordered_map>
+
+bool isHappy(int n) {
+}
+
+int main() {
+	// Test cases
+	// 2, 4, 5, 6, 17, 18, 20 are not happy numbers.
+	// 1, 7, 10, 13, 19, 23, 28, 68 are not happy numbers.
+
+    int testCases[] = {2,4,5,6,17,18,20,68,1,7,10,13,19,23,28};
+
+    for (int n : testCases) {
+        if (isHappy(n)) {
+            std::cout << n << " is a happy number." << std::endl;
+        } else {
+            std::cout << n << " is not a happy number." << std::endl;
+        }
+    }
+
+    return 0;
+}
+