diff --git a/src/main/java/com/ordestiny/tdd/functions/MissingNumber.java b/src/main/java/com/ordestiny/tdd/functions/MissingNumber.java
new file mode 100644
index 0000000..6ea71af
--- /dev/null
+++ b/src/main/java/com/ordestiny/tdd/functions/MissingNumber.java
@@ -0,0 +1,14 @@
+package com.ordestiny.tdd.functions;
+
+class MissingNumber {
+
+    static int getMissingNo(int numberArray[]) {
+
+        int n = numberArray.length;
+        int i, total;
+        total = (n + 1) * (n + 2) / 2;
+        for (i = 0; i < n; i++)
+            total -= numberArray[i];
+        return total;
+    }
+}
\ No newline at end of file
diff --git a/src/test/java/com/ordestiny/tdd/functions/MissingNumberTest.java b/src/test/java/com/ordestiny/tdd/functions/MissingNumberTest.java
new file mode 100644
index 0000000..6a33137
--- /dev/null
+++ b/src/test/java/com/ordestiny/tdd/functions/MissingNumberTest.java
@@ -0,0 +1,27 @@
+package com.ordestiny.tdd.functions;
+
+import org.junit.jupiter.api.Test;
+import static org.junit.jupiter.api.Assertions.*;
+
+class MissingNumberTest {
+
+
+    @Test
+    public void testMissingNumberFail() {
+
+        int a[] = {1, 2, 4, 5, 6};
+        int missedNumber = MissingNumber.getMissingNo(a);
+        // assert
+        assertEquals(7, missedNumber);
+    }
+
+
+    @Test
+    public void testMissingNumberPassed() {
+
+        int a[] = {1, 2, 3, 4, 5, 6, 8};
+        int missedNumber = MissingNumber.getMissingNo(a);
+        // assert
+        assertEquals(7, missedNumber);
+    }
+}
\ No newline at end of file