diff --git a/exercises/standard_library_types/arc1.rs b/exercises/standard_library_types/arc1.rs
index 93a27036..e3a3b52c 100644
--- a/exercises/standard_library_types/arc1.rs
+++ b/exercises/standard_library_types/arc1.rs
@@ -12,25 +12,22 @@
 // Because we are using threads, our values need to be thread-safe.  Therefore,
 // we are using Arc.  We need to make a change in each of the two TODOs.
 
-
 // Make this code compile by filling in a value for `shared_numbers` where the
 // first TODO comment is, and create an initial binding for `child_numbers`
 // where the second TODO comment is. Try not to create any copies of the `numbers` Vec!
 // Execute `rustlings hint arc1` or use the `hint` watch subcommand for a hint.
 
-// I AM NOT DONE
-
 #![forbid(unused_imports)] // Do not change this, (or the next) line.
 use std::sync::Arc;
 use std::thread;
 
 fn main() {
     let numbers: Vec<_> = (0..100u32).collect();
-    let shared_numbers = // TODO
+    let shared_numbers = Arc::new(numbers);
     let mut joinhandles = Vec::new();
 
     for offset in 0..8 {
-        let child_numbers = // TODO
+        let child_numbers = Arc::clone(&shared_numbers);
         joinhandles.push(thread::spawn(move || {
             let sum: u32 = child_numbers.iter().filter(|n| *n % 8 == offset).sum();
             println!("Sum of offset {} is {}", offset, sum);
diff --git a/exercises/standard_library_types/box1.rs b/exercises/standard_library_types/box1.rs
index 66cf00f3..7d730900 100644
--- a/exercises/standard_library_types/box1.rs
+++ b/exercises/standard_library_types/box1.rs
@@ -16,11 +16,9 @@
 //
 // Execute `rustlings hint box1` or use the `hint` watch subcommand for a hint.
 
-// I AM NOT DONE
-
 #[derive(PartialEq, Debug)]
 pub enum List {
-    Cons(i32, List),
+    Cons(i32, Box<List>),
     Nil,
 }
 
@@ -31,13 +29,15 @@ fn main() {
         create_non_empty_list()
     );
 }
-
 pub fn create_empty_list() -> List {
-    todo!()
+    List::Nil
 }
 
-pub fn create_non_empty_list() -> List {
-    todo!()
+pub fn create_non_empty_list() -> Box<List> {
+    Box::new(List::Cons(
+        1,
+        Box::new(List::Cons(2, Box::new(List::Cons(3, Box::new(List::Nil))))),
+    ))
 }
 
 #[cfg(test)]
@@ -51,6 +51,6 @@ mod tests {
 
     #[test]
     fn test_create_non_empty_list() {
-        assert_ne!(create_empty_list(), create_non_empty_list())
+        assert_ne!(create_empty_list(), *create_non_empty_list())
     }
 }
diff --git a/exercises/standard_library_types/cow1.rs b/exercises/standard_library_types/cow1.rs
index 5fba2519..2e4483de 100644
--- a/exercises/standard_library_types/cow1.rs
+++ b/exercises/standard_library_types/cow1.rs
@@ -5,8 +5,6 @@
 // It can enclose and provide immutable access to borrowed data, and clone the data lazily when mutation or ownership is required.
 // The type is designed to work with general borrowed data via the Borrow trait.
 
-// I AM NOT DONE
-
 use std::borrow::Cow;
 
 fn abs_all<'a, 'b>(input: &'a mut Cow<'b, [i32]>) -> &'a mut Cow<'b, [i32]> {
@@ -42,7 +40,7 @@ fn main() {
     let mut input = Cow::from(slice);
     match abs_all(&mut input) {
         // TODO
-        Cow::Borrowed(_) => println!("I own this slice!"),
+        Cow::Owned(_) => println!("I own this slice!"),
         _ => panic!("expected borrowed value"),
     }
 }
diff --git a/exercises/standard_library_types/rc1.rs b/exercises/standard_library_types/rc1.rs
index 9b907fde..b50b5c9d 100644
--- a/exercises/standard_library_types/rc1.rs
+++ b/exercises/standard_library_types/rc1.rs
@@ -5,7 +5,6 @@
 
 // Make this code compile by using the proper Rc primitives to express that the sun has multiple owners.
 
-// I AM NOT DONE
 use std::rc::Rc;
 
 #[derive(Debug)]
@@ -54,17 +53,17 @@ fn main() {
     jupiter.details();
 
     // TODO
-    let saturn = Planet::Saturn(Rc::new(Sun {}));
+    let saturn = Planet::Saturn(Rc::clone(&sun));
     println!("reference count = {}", Rc::strong_count(&sun)); // 7 references
     saturn.details();
 
     // TODO
-    let uranus = Planet::Uranus(Rc::new(Sun {}));
+    let uranus = Planet::Uranus(Rc::clone(&sun));
     println!("reference count = {}", Rc::strong_count(&sun)); // 8 references
     uranus.details();
 
     // TODO
-    let neptune = Planet::Neptune(Rc::new(Sun {}));
+    let neptune = Planet::Neptune(Rc::clone(&sun));
     println!("reference count = {}", Rc::strong_count(&sun)); // 9 references
     neptune.details();
 
@@ -86,12 +85,15 @@ fn main() {
     println!("reference count = {}", Rc::strong_count(&sun)); // 4 references
 
     // TODO
+    drop(earth);
     println!("reference count = {}", Rc::strong_count(&sun)); // 3 references
 
     // TODO
+    drop(mercury);
     println!("reference count = {}", Rc::strong_count(&sun)); // 2 references
 
     // TODO
+    drop(venus);
     println!("reference count = {}", Rc::strong_count(&sun)); // 1 reference
 
     assert_eq!(Rc::strong_count(&sun), 1);