public static function multfract($c, $d, $i, $f)
 {
     Fraction::check($d);
     //checking if denominator is not zero
     Fraction::check($f);
     //checking if denominator is not zero
     $mf = $c / $d * ($i / $f);
     echo $c . "/" . $d . " * " . $i . "/" . $f . " = " . number_format($mf, 2, ',', '') . '<br/>';
 }