diff --git a/exercises/19_smart_pointers/cow1.rs b/exercises/19_smart_pointers/cow1.rs
index 71ac45b3..e9c8294f 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]> {
@@ -49,6 +47,8 @@ mod tests {
         let mut input = Cow::from(&slice[..]);
         match abs_all(&mut input) {
             // TODO
+            Cow::Owned(..) => Err("Mutated"),
+            Cow::Borrowed(..) => Ok(()),
         }
     }
 
@@ -61,6 +61,8 @@ mod tests {
         let mut input = Cow::from(slice);
         match abs_all(&mut input) {
             // TODO
+            Cow::Owned(..) => Ok(()),
+            Cow::Borrowed(..) => Err("Error"),
         }
     }
 
@@ -73,6 +75,8 @@ mod tests {
         let mut input = Cow::from(slice);
         match abs_all(&mut input) {
             // TODO
+            Cow::Owned(c) => Ok(c.push(3)),
+            _ => Err("Error"),
         }
     }
 }
diff --git a/exercises/20_threads/threads1.rs b/exercises/20_threads/threads1.rs
index 80b6def3..c2b0b9f7 100644
--- a/exercises/20_threads/threads1.rs
+++ b/exercises/20_threads/threads1.rs
@@ -1,6 +1,6 @@
 // threads1.rs
 //
-// This program spawns multiple threads that each run for at least 250ms, and
+// INFO:  This program spawns multiple threads that each run for at least 250ms, and
 // each thread returns how much time they took to complete. The program should
 // wait until all the spawned threads have finished and should collect their
 // return values into a vector.
@@ -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};
 
@@ -27,6 +25,8 @@ fn main() {
     let mut results: Vec<u128> = vec![];
     for handle in handles {
         // TODO: a struct is returned from thread::spawn, can you use it?
+        let x = handle.join().unwrap();
+        results.push(x)
     }
 
     if results.len() != 10 {
diff --git a/exercises/20_threads/threads2.rs b/exercises/20_threads/threads2.rs
index 62dad80d..fbdf7df9 100644
--- a/exercises/20_threads/threads2.rs
+++ b/exercises/20_threads/threads2.rs
@@ -1,30 +1,29 @@
 // threads2.rs
 //
-// Building on the last exercise, we want all of the threads to complete their
+// TASK: Building on the last exercise, we want all of the threads to complete their
 // work but this time the spawned threads need to be in charge of updating a
 // shared value: JobStatus.jobs_completed
 //
 // 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;
-
+#[derive(Debug)]
 struct JobStatus {
     jobs_completed: u32,
 }
 
 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
+            let mut status_shared = status_shared.lock().unwrap();
             status_shared.jobs_completed += 1;
         });
         handles.push(handle);
@@ -34,6 +33,6 @@ 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());
     }
 }
diff --git a/exercises/20_threads/threads3.rs b/exercises/20_threads/threads3.rs
index 91006bbc..7e52f981 100644
--- a/exercises/20_threads/threads3.rs
+++ b/exercises/20_threads/threads3.rs
@@ -3,8 +3,6 @@
 // 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;
@@ -28,21 +26,27 @@ impl Queue {
 
 fn send_tx(q: Queue, tx: mpsc::Sender<u32>) -> () {
     let qc = Arc::new(q);
+    let mut t_arc = Arc::new(tx);
+
     let qc1 = Arc::clone(&qc);
+
     let qc2 = Arc::clone(&qc);
 
+    let mut t_arc1 = Arc::clone(&t_arc);
     thread::spawn(move || {
         for val in &qc1.first_half {
             println!("sending {:?}", val);
-            tx.send(*val).unwrap();
+            t_arc1.send(*val).unwrap();
             thread::sleep(Duration::from_secs(1));
         }
     });
 
+    let mut t_arc2 = Arc::clone(&t_arc);
     thread::spawn(move || {
         for val in &qc2.second_half {
             println!("sending {:?}", val);
-            tx.send(*val).unwrap();
+
+            t_arc2.clone().send(*val).unwrap();
             thread::sleep(Duration::from_secs(1));
         }
     });
diff --git a/exercises/21_macros/macros1.rs b/exercises/21_macros/macros1.rs
index 678de6ee..9d0edee3 100644
--- a/exercises/21_macros/macros1.rs
+++ b/exercises/21_macros/macros1.rs
@@ -3,8 +3,6 @@
 // Execute `rustlings hint macros1` or use the `hint` watch subcommand for a
 // hint.
 
-// I AM NOT DONE
-
 macro_rules! my_macro {
     () => {
         println!("Check out my macro!");
@@ -12,5 +10,5 @@ macro_rules! my_macro {
 }
 
 fn main() {
-    my_macro();
+    my_macro!();
 }
diff --git a/exercises/21_macros/macros2.rs b/exercises/21_macros/macros2.rs
index 788fc16a..a87be198 100644
--- a/exercises/21_macros/macros2.rs
+++ b/exercises/21_macros/macros2.rs
@@ -3,14 +3,12 @@
 // Execute `rustlings hint macros2` or use the `hint` watch subcommand for a
 // hint.
 
-// I AM NOT DONE
-
-fn main() {
-    my_macro!();
-}
-
 macro_rules! my_macro {
     () => {
         println!("Check out my macro!");
     };
 }
+
+fn main() {
+    my_macro!();
+}
diff --git a/exercises/21_macros/macros3.rs b/exercises/21_macros/macros3.rs
index b795c149..b5be9c39 100644
--- a/exercises/21_macros/macros3.rs
+++ b/exercises/21_macros/macros3.rs
@@ -5,8 +5,7 @@
 // Execute `rustlings hint macros3` or use the `hint` watch subcommand for a
 // hint.
 
-// I AM NOT DONE
-
+#[macro_use]
 mod macros {
     macro_rules! my_macro {
         () => {
diff --git a/exercises/21_macros/macros4.rs b/exercises/21_macros/macros4.rs
index 71b45a09..1c91f1c0 100644
--- a/exercises/21_macros/macros4.rs
+++ b/exercises/21_macros/macros4.rs
@@ -3,14 +3,12 @@
 // Execute `rustlings hint macros4` or use the `hint` watch subcommand for a
 // hint.
 
-// I AM NOT DONE
-
 #[rustfmt::skip]
 macro_rules! my_macro {
     () => {
         println!("Check out my macro!");
-    }
-    ($val:expr) => {
+    };
+    ( $val:expr) => {
         println!("Look at this other macro: {}", $val);
     }
 }
diff --git a/exercises/22_clippy/clippy1.rs b/exercises/22_clippy/clippy1.rs
index e0c6ce7c4..19f07031 100644
--- a/exercises/22_clippy/clippy1.rs
+++ b/exercises/22_clippy/clippy1.rs
@@ -14,7 +14,7 @@
 use std::f32;
 
 fn main() {
-    let pi = 3.14f32;
+    let pi = f32::consts::PI;
     let radius = 5.00f32;
 
     let area = pi * f32::powi(radius, 2);
diff --git a/temp_36580_ThreadId1 b/temp_36580_ThreadId1
new file mode 100644
index 00000000..e69de29b