diff --git a/exercises/conversions/as_ref_mut.rs b/exercises/conversions/as_ref_mut.rs
index df626e8b..cf1d56e0 100644
--- a/exercises/conversions/as_ref_mut.rs
+++ b/exercises/conversions/as_ref_mut.rs
@@ -9,23 +9,31 @@
 
 // I AM NOT DONE
 
+use std::fmt::Debug;
+
 // Obtain the number of bytes (not characters) in the given argument.
 // TODO: Add the AsRef trait appropriately as a trait bound.
-fn byte_counter<T>(arg: T) -> usize {
+fn byte_counter<T: AsRef<str>>(arg: T) -> usize {
     arg.as_ref().as_bytes().len()
 }
 
 // Obtain the number of characters (not bytes) in the given argument.
 // TODO: Add the AsRef trait appropriately as a trait bound.
-fn char_counter<T>(arg: T) -> usize {
+fn char_counter<T: AsRef<str>>(arg: T) -> usize {
     arg.as_ref().chars().count()
 }
 
 // Squares a number using as_mut().
 // TODO: Add the appropriate trait bound.
-fn num_sq<T : std::ops::Add<Output = T>>(arg: &mut T) {
+// First version that works for mut ref of a num, but it doesn't work if the num is inside something like Box
+// fn num_sq<T : std::ops::Add<Output = T> + AsMut<i32>>(arg: &mut T) {
+fn num_sq<T, U>(arg: &mut T) 
+where 
+    T : AsMut<U> + std::fmt::Debug,   // generalised version where T is a generic container and U is the numeric type
+    U : std::ops::Mul<Output = U> + Copy  
+{
     // TODO: Implement the function body.
-    arg = arg * arg;
+    *arg.as_mut() = *arg.as_mut() * *arg.as_mut(); 
 }
 
 #[cfg(test)]