diff --git a/01-problem/src/main.rs b/01-problem/src/main.rs
index d8930c9..22c551a 100644
--- a/01-problem/src/main.rs
+++ b/01-problem/src/main.rs
@@ -13,7 +13,7 @@ impl CredentialManager {
         }
     }
 
-    pub fn unlock(&self, master_secret: &str) {
+    pub fn unlock(&mut self, master_secret: &str) {
         todo!()
     }
 
@@ -21,11 +21,11 @@ impl CredentialManager {
         todo!()
     }
 
-    pub fn get(&mut self, key: &str) -> Option<&str> {
+    pub fn get(&self, key: &str) -> Option<&str> {
         todo!()
     }
 
-    pub fn lock(&self) {
+    pub fn lock(&mut self) {
         todo!()
     }
 }
diff --git a/02-runtime-check/src/main.rs b/02-runtime-check/src/main.rs
index bf9cd4b..70bfc3d 100644
--- a/02-runtime-check/src/main.rs
+++ b/02-runtime-check/src/main.rs
@@ -15,10 +15,11 @@ impl CredentialManager {
         }
     }
 
-    pub fn unlock(&self, master_secret: &str) -> Result<(), &'static str> {
+    pub fn unlock(&mut self, master_secret: &str) -> Result<(), &'static str> {
         if !self.locked {
             return Err("already unlocked");
         }
+        self.locked = false;
         todo!()
     }
 
@@ -29,17 +30,18 @@ impl CredentialManager {
         todo!()
     }
 
-    pub fn get(&mut self, key: &str) -> Result<Option<&str>, &'static str> {
+    pub fn get(&self, key: &str) -> Result<Option<&str>, &'static str> {
         if self.locked {
             return Err("locked");
         }
         todo!()
     }
 
-    pub fn lock(&self) -> Result<(), &'static str> {
+    pub fn lock(&mut self) -> Result<(), &'static str> {
         if self.locked {
             return Err("already locked");
         }
+        self.locked = true;
         todo!()
     }
 }
diff --git a/03-builder-pattern/src/main.rs b/03-builder-pattern/src/main.rs
index 52b4f6f..e6269d9 100644
--- a/03-builder-pattern/src/main.rs
+++ b/03-builder-pattern/src/main.rs
@@ -29,7 +29,7 @@ impl UnlockedCredentialManager {
         todo!()
     }
 
-    pub fn get(&mut self, key: &str) -> Option<&str> {
+    pub fn get(&self, key: &str) -> Option<&str> {
         todo!()
     }
 
diff --git a/04-typestate/src/main.rs b/04-typestate/src/main.rs
index dd25450..fd6cec9 100644
--- a/04-typestate/src/main.rs
+++ b/04-typestate/src/main.rs
@@ -33,7 +33,7 @@ impl CredentialManager<Unlocked> {
         todo!()
     }
 
-    pub fn get(&mut self, key: &str) -> Option<&str> {
+    pub fn get(&self, key: &str) -> Option<&str> {
         todo!()
     }