请教机警解题思路（急）多谢！

1、一个人月初银行存款为X，本月内他依次存入50、取40、取70、存80，B是表示本月内的任一次存款帐户余额，求X 与B 的关系？

2、1/33+1/34+-----1/64的取值范围？

3、a certain junior class has 1000 student and a certain senior class has 800 student. among these students ,there are 60 sibling pairs.each containing of 1 junior and 1 senior.if 1 student is to be selected at random from each class,what is the probility that the 2 students selected will be a sibling pair?

4、5个字母DIGIT，可以组成一个STRING。如DIGIT，两个II不相邻，可以组成多少STRING。？