mutable vars description update

challenges/mutable-variables/description.md

@@ -0,0 +1,42 @@
+## Mutable Variables Challenge
+
+Declaring and manipulating variables in programming is a fundamental concept that allows you to store and modify data. Variables in Rust are **immutable by default**, but you can make them mutable using the `mut` keyword.
+
+In this challenge, you will declare and use **mutable variables in Rust**. You will be given a function where you need to declare variables, modify their values, and perform specific operations.
+
+## Your task
+
+In this challenge, you need to declare two mutable variables inside the function using the `let mut` keyword:
+
+- `x` with an initial value of `5`
+- `y` with an initial value of `10`
+
+Then, perform the following operations:
+
+1. Increment `x` by `10`
+2. Multiply `y` by `2`
+
+Finally, calculate the sum of `x` and `y` and return the result.
+
+## Requirements
+
+- Declare two mutable variables, `x` and `y`, and assign them initial values.
+- Increment `x` by `10`.
+- Multiply `y` by `2`.
+- Calculate the sum of `x` and `y`.
+- Return the calculated sum.
+
+## Example
+
+```rust
+let result = manipulate_variables();
+assert_eq!(result, 35);
+```
+
+## Hints
+
+- Use the `let mut` keyword to declare **mutable variables**.
+- Use the `+=` operator to increment a variable.
+- Use the `\*=` operator to multiply a variable.
+- Calculate the sum of `x` and `y` using the `+` operator.
+- Either use a `return` statement or an **expression at the end without a semicolon** to return the result.