Problem 16 of Project Euler:

What is the sum of the digits of the number 2¹⁰⁰⁰?

This is a pretty straightforward problem to solve. I don't even get to draw any goofy pictures to make the logic clear.

The idea is to compute the number, split it into its individual digits and add them all together. Given a number n here's the F# code to extract each digit:

let rec digits n =

    seq {

        if n > 0I then

            yield n%10I

            yield! digits (n/10I)

    }

The desired answer is:

digits (2I ** 1000) |> Seq.sum |> printfn "%A"