Smallest number divisible by each of 1 to 20:

 

First cut (takes almost an hour to execute!):

let isFactor n d = n % d = 0

{1..Int32.MaxValue}

|> Seq.find (fun n -> [1..20] |> List.forall (isFactor n))

 

Second cut (takes a millisecond!):

 

let rec gcd a b = if b = 0 then a else gcd b (a % b)

[1..20] |> List.reduceBack (fun a b -> (a / (gcd a b)) * b)