diff --git a/src/main/scala/ninetyNineScalaProblems/Problem28.scala b/src/main/scala/ninetyNineScalaProblems/Problem28.scala
deleted file mode 100644
index 62fa0e8..0000000
--- a/src/main/scala/ninetyNineScalaProblems/Problem28.scala
+++ /dev/null
@@ -1,7 +0,0 @@
-package ninetyNineScalaProblems.lists
-
-import scala.annotation.tailrec
-
-object Problem28:
-    def lsort(list: List[List[Any]]): List[Any] =
-        list.sortWith((a, b) => a.length < b.length)
\ No newline at end of file
diff --git a/src/main/scala/ninetyNineScalaProblems/arithmetic/Problem31.scala b/src/main/scala/ninetyNineScalaProblems/arithmetic/Problem31.scala
new file mode 100644
index 0000000..613cc88
--- /dev/null
+++ b/src/main/scala/ninetyNineScalaProblems/arithmetic/Problem31.scala
@@ -0,0 +1,21 @@
+package ninetyNineScalaProblems.arithmetic
+
+import scala.annotation.tailrec
+import scala.math.sqrt
+
+object Problem31:
+    def isPrime(int: Int): Boolean =
+        @tailrec
+        def loop(cursor: Int): Boolean =
+            cursor match
+                case c if c >= sqrt(int).toInt + 1 =>
+                    true
+                case c if int % c == 0 =>
+                    false
+                case _ =>
+                    loop(cursor + 1)
+        int match
+            case int if int < 2 =>
+                false
+            case _ =>
+                loop(2)
\ No newline at end of file
diff --git a/src/test/scala/ninetyNineScalaProblems/Problem28Suite.scala b/src/test/scala/ninetyNineScalaProblems/Problem28Suite.scala
deleted file mode 100644
index 1bf47df..0000000
--- a/src/test/scala/ninetyNineScalaProblems/Problem28Suite.scala
+++ /dev/null
@@ -1,15 +0,0 @@
-package ninetyNineScalaProblems.lists
-
-class Problem28Suite extends munit.FunSuite:
-    List(
-        (List(List(1, 2), List(1, 2, 3, 4), List(1, 2, 3)), List(List(1, 2), List(1, 2, 3), List(1, 2, 3, 4))),
-        (List(List(1, 2), List(1, 2, 3), List(1, 2, 3, 4)), List(List(1, 2), List(1, 2, 3), List(1, 2, 3, 4))),
-        (List(), List()),
-        (List(List(1)), List(List(1))),
-        (List(List(1), List(1)), List(List(1), List(1))),
-        (List(List(1, 2, 3), List(4, 5), List(6, 7, 8), List(4, 5), List(9, 10, 11, 12), List(13, 14), List(15)),
-         List(List(15), List(4, 5), List(4, 5), List(13, 14), List(1, 2, 3), List(6, 7, 8), List(9, 10, 11, 12))),
-    ).foreach:
-        case (list, expected) =>
-            test(s"lsort returns [$expected]"):
-                assertEquals(Problem28.lsort(list), expected)
\ No newline at end of file
diff --git a/src/test/scala/ninetyNineScalaProblems/arithmetic/Problem31Suite.scala b/src/test/scala/ninetyNineScalaProblems/arithmetic/Problem31Suite.scala
new file mode 100644
index 0000000..886376d
--- /dev/null
+++ b/src/test/scala/ninetyNineScalaProblems/arithmetic/Problem31Suite.scala
@@ -0,0 +1,16 @@
+package ninetyNineScalaProblems.arithmetic
+
+class Problem31Suite extends munit.FunSuite:
+    List(
+        (1, false),
+        (0, false),
+        (-1, false),
+        (2, true),
+        (3, true),
+        (4, false),
+        (5, true),
+        (71, true),
+    ).foreach:
+        case (int, expected) =>
+            test(s"isPrime returns [$expected]"):
+                assertEquals(Problem31.isPrime(int), expected)
\ No newline at end of file