Complete variables section

exercises/variables/variables1.rs

@@ -2,7 +2,7 @@
 // Make me compile! Scroll down for hints :)
 
 fn main() {
-    x = 5;
+    let x = 5;
     println!("x has the value {}", x);
 }
 

exercises/variables/variables2.rs

@@ -2,7 +2,7 @@
 // Make me compile! Scroll down for hints :)
 
 fn main() {
-    let x;
+    let x: u32 = 9;
     if x == 10 {
         println!("Ten!");
     } else {

exercises/variables/variables3.rs

@@ -2,7 +2,7 @@
 // Make me compile! Scroll down for hints :)
 
 fn main() {
-    let x = 3;
+    let mut x = 3;
     println!("Number {}", x);
     x = 5;
     println!("Number {}", x);

exercises/variables/variables4.rs

@@ -2,7 +2,7 @@
 // Make me compile! Scroll down for hints :)
 
 fn main() {
-    let x: i32;
+    let x: i32 = 69;
     println!("Number {}", x);
 }