diff --git a/exercises/19_smart_pointers/cow1.rs b/exercises/19_smart_pointers/cow1.rs
index fcd3e0bb..5816e5c9 100644
--- a/exercises/19_smart_pointers/cow1.rs
+++ b/exercises/19_smart_pointers/cow1.rs
@@ -12,8 +12,6 @@
 //
 // Execute `rustlings hint cow1` or use the `hint` watch subcommand for a hint.
 
-// I AM NOT DONE
-
 use std::borrow::Cow;
 
 fn abs_all<'a, 'b>(input: &'a mut Cow<'b, [i32]>) -> &'a mut Cow<'b, [i32]> {
@@ -48,7 +46,8 @@ mod tests {
         let slice = [0, 1, 2];
         let mut input = Cow::from(&slice[..]);
         match abs_all(&mut input) {
-            // TODO
+            Cow::Borrowed(_) => Ok(()),
+            _ => Err("Expected borrowed value"),
         }
     }
 
@@ -60,7 +59,8 @@ mod tests {
         let slice = vec![0, 1, 2];
         let mut input = Cow::from(slice);
         match abs_all(&mut input) {
-            // TODO
+            Cow::Owned(_) => Ok(()),
+            _ => Err("Expected owned value"),
         }
     }
 
@@ -72,7 +72,8 @@ mod tests {
         let slice = vec![-1, 0, 1];
         let mut input = Cow::from(slice);
         match abs_all(&mut input) {
-            // TODO
+            Cow::Owned(_) => Ok(()),
+            _ => Err("Expected owned value"),
         }
     }
 }
diff --git a/exercises/20_threads/threads1.rs b/exercises/20_threads/threads1.rs
index 80b6def3..a4c89854 100644
--- a/exercises/20_threads/threads1.rs
+++ b/exercises/20_threads/threads1.rs
@@ -8,8 +8,6 @@
 // Execute `rustlings hint threads1` or use the `hint` watch subcommand for a
 // hint.
 
-// I AM NOT DONE
-
 use std::thread;
 use std::time::{Duration, Instant};
 
@@ -26,7 +24,7 @@ fn main() {
 
     let mut results: Vec<u128> = vec![];
     for handle in handles {
-        // TODO: a struct is returned from thread::spawn, can you use it?
+        results.push(handle.join().unwrap());
     }
 
     if results.len() != 10 {
diff --git a/exercises/20_threads/threads2.rs b/exercises/20_threads/threads2.rs
index 62dad80d..ec5c61b2 100644
--- a/exercises/20_threads/threads2.rs
+++ b/exercises/20_threads/threads2.rs
@@ -7,9 +7,7 @@
 // Execute `rustlings hint threads2` or use the `hint` watch subcommand for a
 // hint.
 
-// I AM NOT DONE
-
-use std::sync::Arc;
+use std::sync::{Arc, Mutex};
 use std::thread;
 use std::time::Duration;
 
@@ -18,14 +16,14 @@ struct JobStatus {
 }
 
 fn main() {
-    let status = Arc::new(JobStatus { jobs_completed: 0 });
+    let status = Arc::new(Mutex::new(JobStatus { jobs_completed: 0 }));
     let mut handles = vec![];
     for _ in 0..10 {
         let status_shared = Arc::clone(&status);
         let handle = thread::spawn(move || {
             thread::sleep(Duration::from_millis(250));
             // TODO: You must take an action before you update a shared value
-            status_shared.jobs_completed += 1;
+            status_shared.lock().unwrap().jobs_completed += 1;
         });
         handles.push(handle);
     }
@@ -34,6 +32,6 @@ fn main() {
         // TODO: Print the value of the JobStatus.jobs_completed. Did you notice
         // anything interesting in the output? Do you have to 'join' on all the
         // handles?
-        println!("jobs completed {}", ???);
+        println!("jobs completed {}", status.lock().unwrap().jobs_completed);
     }
 }
diff --git a/exercises/20_threads/threads3.rs b/exercises/20_threads/threads3.rs
index 91006bbc..66af420a 100644
--- a/exercises/20_threads/threads3.rs
+++ b/exercises/20_threads/threads3.rs
@@ -1,10 +1,8 @@
 // threads3.rs
-//
 // Execute `rustlings hint threads3` or use the `hint` watch subcommand for a
+//
 // hint.
 
-// I AM NOT DONE
-
 use std::sync::mpsc;
 use std::sync::Arc;
 use std::thread;
@@ -26,32 +24,36 @@ impl Queue {
     }
 }
 
-fn send_tx(q: Queue, tx: mpsc::Sender<u32>) -> () {
-    let qc = Arc::new(q);
-    let qc1 = Arc::clone(&qc);
-    let qc2 = Arc::clone(&qc);
-
+fn send_tx(q: Arc<Queue>, tx: mpsc::Sender<u32>) {
+    let q1 = Arc::clone(&q);
+    let tx1 = tx.clone();
     thread::spawn(move || {
-        for val in &qc1.first_half {
+        for val in &q1.first_half {
             println!("sending {:?}", val);
-            tx.send(*val).unwrap();
+            tx1.send(*val).unwrap();
             thread::sleep(Duration::from_secs(1));
         }
-    });
+    })
+    .join()
+    .unwrap();
 
+    let q2 = Arc::clone(&q);
+    let tx2 = tx.clone();
     thread::spawn(move || {
-        for val in &qc2.second_half {
+        for val in &q2.second_half {
             println!("sending {:?}", val);
-            tx.send(*val).unwrap();
+            tx2.send(*val).unwrap();
             thread::sleep(Duration::from_secs(1));
         }
-    });
+    })
+    .join()
+    .unwrap();
 }
 
 #[test]
 fn main() {
     let (tx, rx) = mpsc::channel();
-    let queue = Queue::new();
+    let queue = Arc::new(Queue::new());
     let queue_length = queue.length;
 
     send_tx(queue, tx);
@@ -63,5 +65,5 @@ fn main() {
     }
 
     println!("total numbers received: {}", total_received);
-    assert_eq!(total_received, queue_length)
 }
+