diff --git a/src/main/scala/ninetyNineScalaProblems/Problem21.scala b/src/main/scala/ninetyNineScalaProblems/Problem21.scala
new file mode 100644
index 0000000..f6403e0
--- /dev/null
+++ b/src/main/scala/ninetyNineScalaProblems/Problem21.scala
@@ -0,0 +1,5 @@
+package ninetyNineScalaProblems
+
+object Problem21:
+    def insertAt(value: Any, n: Int, list: List[Any]): List[Any] =
+        Problem18.slice(0, n, list) ::: value :: Problem18.slice(n, list.length, list)
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diff --git a/src/test/scala/ninetyNineScalaProblems/Problem21Suite.scala b/src/test/scala/ninetyNineScalaProblems/Problem21Suite.scala
new file mode 100644
index 0000000..81489d0
--- /dev/null
+++ b/src/test/scala/ninetyNineScalaProblems/Problem21Suite.scala
@@ -0,0 +1,15 @@
+package ninetyNineScalaProblems
+
+import scala.compiletime.ops.boolean
+
+class Problem21Suite extends munit.FunSuite:
+    List(
+        ((1, 0, List(1, 2, 3, 4)), List(1, 1, 2, 3, 4)),
+        ((1, 1, List(1, 2, 3, 4)), List(1, 1, 2, 3, 4)),
+        ((1, 3, List(1, 2, 3, 4)), List(1, 2, 3, 1, 4)),
+        ((1, 0, List(1)), List(1, 1)),
+        ((1, 0, List()), List(1)),
+    ).foreach:
+        case ((value, n, list), expected) =>
+            test(s"insertAt returns [$expected]"):
+                assertEquals(Problem21.insertAt(value, n, list), expected)
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