public class Euler493 {
    private static double chooseRatio(int a, int b, int draws) {
        if (draws == 0)
            return 1.0;
        double ratio = 1.0;
        for (int i = 0; i < draws; ++i) {
            ratio *= (double) (a - i) / (double) (b - i);
        }
        return ratio;
    }

    public static void main(String[] args) {
        double missOne = chooseRatio(60, 70, 20);
        double ans = 7.0 * (1.0 - missOne);
        System.out.printf(java.util.Locale.US, "%.9f\n", ans);
    }
}