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/>';
}