findSum n = f 5 n + f 3 n - f 15 n
    where f d n = let a = (n `div` d) in (a * d * succ a) `div` 2
findSum n = f 5 n + f 3 n - f 15 n
    where f d n = let a = (n `div` d) in (a * d * succ a) `div` 2
findSum n = f 5 n + f 3 n - f 15 n
    where f d n = let a = (n `div` d) in (a * d * succ a) `div` 2