diff --git a/src/main/scala/com/chobostudy/loustler/error/Option.scala b/src/main/scala/com/chobostudy/loustler/error/Option.scala
index c378398..1624366 100644
--- a/src/main/scala/com/chobostudy/loustler/error/Option.scala
+++ b/src/main/scala/com/chobostudy/loustler/error/Option.scala
@@ -38,3 +38,7 @@ trait Option[+A] {
 
 case class Some[+A](get: A) extends Option[A]
 case object None extends Option[Nothing]
+
+object Option {
+  def apply[T](t: T): Option[T] = if (t == null) None else Some(t)
+}
diff --git a/src/main/scala/com/chobostudy/loustler/error/SolutionP68.scala b/src/main/scala/com/chobostudy/loustler/error/SolutionP68.scala
new file mode 100644
index 0000000..04c7c8e
--- /dev/null
+++ b/src/main/scala/com/chobostudy/loustler/error/SolutionP68.scala
@@ -0,0 +1,24 @@
+package com.chobostudy.loustler.error
+
+import com.chobostudy.loustler.error.Option
+
+/**
+  * @author loustler
+  * @since 11/03/2017 21:19
+  */
+trait SolutionP68 {
+  def mean(xs: Seq[Double]): Option[Double] = {
+    Option(xs.sum / xs.size)
+  }
+
+  // exercise 4.2
+  def variance(xs: Seq[Double]): Option[Double] = {
+    Option(xs).flatMap(x => {
+      val meanV = mean(xs).getOrElse(0.0)
+
+      mean(xs.map(d => math.pow(meanV - d, 2)))
+    })
+  }
+}
+
+object SolutionP68 extends SolutionP68
diff --git a/src/test/scala/com/chobostudy/loustler/error/SolutionP68Test.scala b/src/test/scala/com/chobostudy/loustler/error/SolutionP68Test.scala
new file mode 100644
index 0000000..ab87f52
--- /dev/null
+++ b/src/test/scala/com/chobostudy/loustler/error/SolutionP68Test.scala
@@ -0,0 +1,20 @@
+package com.chobostudy.loustler.error
+
+import org.scalatest.FunSuite
+
+/**
+  * @author loustler
+  * @since 11/03/2017 21:52
+  */
+class SolutionP68Test extends FunSuite {
+  test("variance should be correct") {
+    val seq = List(1.0, 1.1, 1.2, 1.3, 1.4)
+    val mean = seq.sum / seq.size
+    val variance = seq.map(d => Math.pow(mean - d, 2)).sum / seq.size
+    val expect = Option(variance)
+
+    val result = SolutionP68.variance(seq)
+
+    assert(result == expect)
+  }
+}