$ perl main.pl
$a  = 19, $b = 13, $c = 17 ，$d = 7
($a + $b) * $c / $d  = 77.7142857142857
(($a + $b) * $c) / $d  = 77.7142857142857
($a + $b) * ($c / $d )  = 77.7142857142857
$a + ($b * $c )/ $d  = 50.5714285714286