let face (x:int) :string =
let remainder = x % 13
if remainder = 1 then "Ace"
elif remainder = 11 then "Jack"
elif remainder = 12 then "Queen"
elif remainder = 0 then "King"
else string remainder

let suit (x:int) :string =
if x < 13 then "Hearts"
elif x > 13 && x < 27 then "Diamonds"
elif x > 26 && x < 40 then "Clubs"
else "Spades"

[<EntryPoint>]
let main argv =

let mutable keeplooping = true

while keeplooping do
printfn "Enter Card Number"
let x = Console.ReadLine()
printfn "%s of %s" (face (int x)) (suit (int x))

exit 100

No explanation as yet, I'm still getting to grips with this myself.