Problem 1: Add all the natural numbers below one thousand that are multiples of 3 or 5.

Solutions (a)-(c) take the same approach but use different Seq function applications. Every integer from 1 to 999 is tested.

let problem1a =
    {1..999}
    |> Seq.filter (fun i -> i % 3 = 0 || i % 5 = 0)
    |> Seq.sum

let problem1b =
    {1..999}
    |> Seq.fold (fun acc i -> if i % 3 = 0 || i % 5 = 0 then acc + i else acc) 0
    
let problem1c =    
    {1..999}
    |> Seq.sumBy (fun i -> if i % 3 = 0 || i % 5 = 0 then i else 0)

Solution (d) uses inclusion/exclusion instead of an exhaustive search.

let problem1d = 
    let a = Seq.append {0..3..999} {0..5..999} |> Seq.sum
    let b = {0..15..999} |> Seq.sum
    a - b    

I like solution (a) the best since it's the clearest.