$highest =0;
while($row = mysql_fetch_array( $result )) {

if(($row['yanlis'] + $row['dogru']) > '0') {     //to avoid dividing by zero
$per = ($row['dogru'] / ($row['dogru'] + $row['yanlis']))*'100';
if ($per>$highest) {
 $highest = $per;
}
//echo $per, "<br>";

}
}
echo floor($highest);