Is it okay to declare variable inside if statment ?

Yes it's fine - you can declare a variable within any set of curly brackets, and it has it's advantages, because the variable goes out of scope at the closing bracket. And when it goes out of scope, it cannot be referenced.

So if for example you had a pointer which you allocated memory, used it and then deallocated it, declaring it at the top of the if and deallocating the memory at the bottom means that you cannot accidently use the deallocated pointer outside the block.

Your example is pretty trivial, but yes, it's allowed, and often a good idea!

Yes, but that will not work in your case. if block itself is a scope, and you define a scope and the variables are only accessible inside the scope, not outside, and not at the boundary either.

So, I think something you would want to do would be,

cin >> y;

if (y > 10)
{
   double x = y * 15;

This would work, and will test if the value of y is greater. Why I think this would be the right approach? Because, you just took input in the y variable, and you would now need to test if the input from user was more than 10.

Otherwise, you can do this,

cin >> y;

double x = 0; // Initialized with 0, to avoid garbage, and to make my next sentence true
if (x > 10)
{
    // x has been already defined
    x = y * 15;

But notice, that now your if block would never execute, because it won't be more than 10 and thus if block would never executed.

Scope - cppreference.com[^]

Sure. Just don't expect to be able to use that variable outside the if block.

What you're looking at is "scope". This is the concept is visibility for variables and methods. Read up on it here: Variable and Method Scope in Microsoft .NET[^].