How to Declare and Define an double data type variable in F# ?

// AbundantCode F# Tutorials and Code snippets
open System
[<EntryPoint>]
let main argv = 
    //How to Declare and Define an double data type variable in F# ?
    let doubledatatypevariable = 10.5
    Console.WriteLine(doubledatatypevariable)
    let retval = Console.ReadLine()
    0 // return an integer exit code