diff --git a/CHANGELOG.md b/CHANGELOG.md
index e2acf6c6..2132b648 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -9,6 +9,7 @@
 - New argument `--no-editor` to disable automatic opening of the current file in VS Code or Zellij
 - New argument `--edit-cmd` to communicate with an editor running in a different process to open the current exercise
 - Show the file link of the current exercise when running `rustlings hint` and `rustlings reset`
+- Add a new `traits6` exercise.
 
 ### Fixed
 
diff --git a/dev/Cargo.toml b/dev/Cargo.toml
index 66bc1dfe..4bcea6ea 100644
--- a/dev/Cargo.toml
+++ b/dev/Cargo.toml
@@ -126,6 +126,8 @@ bin = [
   { name = "traits4_sol", path = "../solutions/15_traits/traits4.rs" },
   { name = "traits5", path = "../exercises/15_traits/traits5.rs" },
   { name = "traits5_sol", path = "../solutions/15_traits/traits5.rs" },
+  { name = "traits6", path = "../exercises/15_traits/traits6.rs" },
+  { name = "traits6_sol", path = "../solutions/15_traits/traits6.rs" },
   { name = "quiz3", path = "../exercises/quizzes/quiz3.rs" },
   { name = "quiz3_sol", path = "../solutions/quizzes/quiz3.rs" },
   { name = "lifetimes1", path = "../exercises/16_lifetimes/lifetimes1.rs" },
diff --git a/exercises/15_traits/traits6.rs b/exercises/15_traits/traits6.rs
new file mode 100644
index 00000000..0c873567
--- /dev/null
+++ b/exercises/15_traits/traits6.rs
@@ -0,0 +1,33 @@
+// TODO: Design a trait that enables you to calculate the digit sum over both an
+// integer as well as a string.
+
+// TODO: Change this function signature so it works with both integers and
+// strings, based on your trait.
+fn calculate_digit_sum(number: ???) -> usize {
+    // TODO: Calculate the digit sum of `number`.
+}
+
+fn main() {
+    // You can optionally experiment here.
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn integer() {
+        assert_eq!(calculate_digit_sum(0), 1);
+        assert_eq!(calculate_digit_sum(1), 1);
+        assert_eq!(calculate_digit_sum(1235), 11);
+        assert_eq!(calculate_digit_sum(789), 24);
+    }
+
+    #[test]
+    fn string() {
+        assert_eq!(calculate_digit_sum(""), 1);
+        assert_eq!(calculate_digit_sum("1"), 1);
+        assert_eq!(calculate_digit_sum("1235"), 11);
+        assert_eq!(calculate_digit_sum("789"), 24);
+    }
+}
diff --git a/rustlings-macros/info.toml b/rustlings-macros/info.toml
index 637283d2..6edc3ac8 100644
--- a/rustlings-macros/info.toml
+++ b/rustlings-macros/info.toml
@@ -793,6 +793,16 @@ replacing `???` with 'impl [what goes here?] + [what goes here?]'.
 Related section in The Book:
 https://doc.rust-lang.org/book/ch10-02-traits.html#specifying-multiple-trait-bounds-with-the--syntax"""
 
+[[exercises]]
+name = "traits6"
+dir = "15_traits"
+hint = """
+Remember that you can implement a trait you defined yourself on foreign types,
+i.e. ones from the standard library or third-party libraries. A trait that only
+serves the purpose of adding additional functionality to foreign types is often
+referred to as an "extension trait". Define such a trait and implement it for
+`usize` and `&str`."""
+
 # QUIZ 3
 
 [[exercises]]
diff --git a/solutions/15_traits/traits6.rs b/solutions/15_traits/traits6.rs
new file mode 100644
index 00000000..de620cd1
--- /dev/null
+++ b/solutions/15_traits/traits6.rs
@@ -0,0 +1,53 @@
+trait Digits {
+    fn digits(&self) -> Vec<usize>;
+}
+
+impl Digits for usize {
+    fn digits(&self) -> Vec<usize> {
+        let mut res = Vec::new();
+        let mut temp = *self;
+        while temp != 0 {
+            res.push(temp % 10);
+            temp /= 10;
+        }
+        res.reverse();
+        res
+    }
+}
+
+impl Digits for &str {
+    fn digits(&self) -> Vec<usize> {
+        self.chars()
+            .map(|c| c.to_digit(10).unwrap() as usize)
+            .collect()
+    }
+}
+
+fn calculate_digit_sum(number: impl Digits) -> usize {
+    number.digits().into_iter().sum()
+}
+
+fn main() {
+    // You can optionally experiment here.
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn integer() {
+        assert_eq!(calculate_digit_sum(0), 1);
+        assert_eq!(calculate_digit_sum(1), 1);
+        assert_eq!(calculate_digit_sum(1235), 11);
+        assert_eq!(calculate_digit_sum(789), 24);
+    }
+
+    #[test]
+    fn string() {
+        assert_eq!(calculate_digit_sum(""), 1);
+        assert_eq!(calculate_digit_sum("1"), 1);
+        assert_eq!(calculate_digit_sum("1235"), 11);
+        assert_eq!(calculate_digit_sum("789"), 24);
+    }
+}