18 numbers (can be the same) are arranged in a row in ascending order. The average of the first 10 numbers is 28.5, the average of the last 10 numbers is 31.2, and the average of the 18 numbers is 30. What is the fifth number.
18个数(可以有相同的)按从小到大的顺序排成一排．前10个数的平均数是28.5，后10个数的平均数是31.2，18个数的平均数为30．第五个数是多少．
<result>28.5</result>