Add variables example

variables/Cargo.toml

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+[package]
+name = "variables"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]

variables/src/main.rs

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+const THREE_HOURS_IN_SECONDS: u32 = 60 * 60 * 3;
+
+fn main() {
+    // Mutable variable can have value changed at will
+    let mut x = 5;
+    println!("The value of x is: {x}");
+    x = 6;
+    println!("The value of x is: {x}");
+
+    let y = 5;
+    // Can shadow and reuse the variable `y` by using `let` again
+    let y = y + 1;
+    {
+        // New `y` variable exists for life of the scope
+        let y = y * 2;
+        println!("The value of y in the inner scope is: {y}");
+    }
+
+    // `y` variable returns to value outside of the scope (6)
+    println!("The value of y is: {y}");
+
+    // We can use shadowing to change the variable's type
+    let spaces = "   ";
+    let spaces = spaces.len();
+}