智力测试,look at the numbers below .What is the number after 48?

问题描述:

智力测试,look at the numbers below .What is the number after 48?
9,12,21,48

9+3=12
12+9=21
21+27=48
48+81=129
第n个数都是前一个数加上3的n-1次,
所以48后面一个数是129