SAT OG 2道数学题,牛人帮帮忙啦～～

题目

SAT OG 2道数学题,牛人帮帮忙啦～～
P598 就是有5个正方形卡片,其中一张不能放在最后,问有几种安排方式?
a,3a~~~
the first term in the sequence above is a,and each term after the first is 3 times the preceding term,if the sum of first 5 terms is 605,what is the value of a?我怎么算都是5啊,可答案是6!

答案

1.A5-A4 =96
5 4
2.
5,15,45,135,405,明显答案错 不可能6的倍数最终结果是5的