tuples.ml

let exchange x =
    let ones = (x mod 10) in
    let tens = (x - ones) / 10 in
    (ones * 10) + tens;;

let is_valid_answer (grand_father_age, grand_son_age) =
    let four_multiply = (grand_father_age = (grand_son_age * 4)) in
    let ex_f = exchange grand_father_age in
    let ex_s = exchange grand_son_age in
    let three_mult = (ex_s = (ex_f * 3)) in
    four_multiply && three_mult;;


let rec find (max_father, min_son) =
    match (max_father, min_son) with
    | (f, s) when f < (s * 4) -> (-1, -1)
    | (f, s) when (is_valid_answer (f, s)) -> (f, s)
    | _ -> find (max_father, min_son + 1);;