所求和为第三列和第四列乘积的总和
$("tr td:nth-child(3)").each(function(i){
var abc=parseInt($(this).html().replace("$",""))
//alert(abc);
$("tr td:nth-child(4)").each(function(j){
if(i==j){
third+=abc*parseInt($(this).html());
}
})
$(".costs").html(\'total costs:\'+third+"$");
});