private function multiplyRational(Rational $r): Rational
    {
        $w = $this->whole;
        $n = $this->numerator;
        $d = $this->denominator;

        $w2 = $r->whole;
        $n2 = $r->numerator;
        $d2 = $r->denominator;

        $new_w = $w * $w2;
        $new_n = $w * $n2 * $d + $w2 * $n * $d2 + $n2 * $n;
        $new_d = $d * $d2;

        return new Rational($new_w, $new_n, $new_d);
    }