实用：
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Document</title>
</head>
<body>
<script>
var a=prompt("输入头总数");
var b=prompt("输入脚总数");
//根据m+n=a;2m+4n=b; 则推出
var m=(4*a-b)/2;
var n=(b-2*a)/2;
document.write("那么鸡有",m,"只","<br>");
document.write("兔有",n,"只","<br>");
//alert("那么鸡有",m,"只","<br>");
</script>
</body>
</html>
循环：
// 鸡兔同笼
//1.
var result = null
for (var i = 0; i < 35; i++) {
for (var j = 0; j < 35; j++) {
if ((i * 2) + (j * 4) == 94 && i + j == 35) {
result = '鸡' + i + '兔' + j;
break
}
}
}
// 2.(最佳)
var result = null
for (var i = 0; i < 35; i++) {
var j = 35 - i;
if ((i * 2) + (j * 4) == 94) {
result = '鸡' + i + '兔' + j;
break
}
}
//同2
var result = null,
i = 0
while (i < 35) {
var j = 35 - i;
if ((i * 2) + (j * 4) == 94) {
result = '鸡' + i + '兔' + j;
break
}
i++
}