[求助]高手帮忙解几个题...后天考试...

PREP上的

1. before being simplified the instruction for computing income tax in country R were to add 2 percent of one's annual income to the average of 100 units of country R's currency and 1 precent of one's annual income which of the following represents the simplified formula for computing the income tax in country R's currency, for a person in that country whose annual income is i?

answer: 50+i/40

2. if each term in the sum A1+A2+.....+An is either 7 or 77 and the sum equals 350,which of following be equal to n.

answer:40

3. a certain stock exchange designates each stock with a one-, two-. three-. letter cod, where each letter is selected from the 26 letters of the aphabet. if the letters may be repeated and if the same letters used in a different order constitute a different code, how many different stocks is it possible to uniquely designates with these codes?

answer: 18278...

谢谢

回复第2题：

只有当N=10或20或30或40等 10的倍数的时候 ，各项加和才会等于350

记得当时做的时候先把不是10的倍数的选项排除。

lz给题时最好把其他选项也写出来，PS的题很多是在选项之间用排除法解出

的 

回复第3题：

题目意思是股票可以表示为 one-letter cod 或者 two-letter cod.或者 three-letter cod,

也就是说，一支股票可以 表示成如下形式：C 或者CD或者 AAA

则表示成one-letter cod  有26种方法 C（1,26）=26

表示成TWO-letter cod  有 C（1,26）* C（1,26）=26*26种方法

表示成THREE-letter cod 有26*26*26种方法

则总共有 26+26*26+26*26*26=18278种方法

回复第1题：

add
        2 percent of one's annual income
        to the average of (100 units of country R's currency
        and1 precent of one's annual income)

2%*i  +  (100 + 1%*i
        )/2  =  50 + i/40