A centipede with 40 feet and a dragon with 9 heads are in the same cage. There are 50 heads and 220 feet in total. If each centipede has one head, how many feet does each dragon have?
40只脚的蜈蚣和9个头的龙同在一个笼子里，共有50个头和220只脚，若每只蜈蚣有一个头，则每条龙有多少只脚．
<result>4</result>