docs(variables): fixing the structure of the readmes and adding the hints section

subjects/mobile-dev/variables/README.md

@@ -2,7 +2,17 @@
 
 ### Instructions
 
-You can declare variables in two ways. The type can be implicitly detected using the `var` keyword, or explicitly by declaring the variable's type:
+Declare and initialize following variables:
+
+- `obj` of type `Object` containing any value;
+- `planet` of type `String` containing planet's name you live on;
+- `year` of type `int` containing current year;
+- `lucky` of type `bool` containing true of false (you decide);
+- `pi` which is a constant of type `double` containing the value of pi with 2 decimal points;
+
+### Hints
+
+There are two ways to declare variables in Dart:
 
 ```dart
 var strPatrick = 'Patrick';
@@ -12,12 +22,7 @@ var strPatrick = 'Patrick';
 String strSpongebob = 'Spongebob';
 ```
 
-Declare and initialize the following variables:
-
-- `obj` of type `Object`, containing any value.
-- `planet` of type `String`, containing the name of the planet you live on.
-- `year` of type `int`, containing the current year.
-- `lucky` of type `bool`, containing `true` or `false` (you decide).
-- constant `pi` of type `double`, containing pi value with 2 decimal places
+- The first way declares using `var` which detects variable types automatically.
+- The second way explicitly declares the variable's type using the appropriate keyword, such as `String` for strings, as shown in the example.
 
-> No main is needed.
+> Note: main is not needed!