200-710 Question #111: Real Exam Question with Answer & Explanation

What is the output of the following code?

class Number {
 private static $sv = 10;
 private $v;
 public function __construct($v) { $this->v = $v; }
 public function mul() {
 return static function ($x) {
 return isset($this->v) ? $this->v*$x : self::$sv*$x;
 };
 }
}
$one = new Number(1);
$two = new Number(2);
$double = $two->mul();
$x = Closure::bind($double, null, 'Number');
echo $x(5);