[求助]GWD数学！

数据充分题（选项省略）

In triangle ABC, point X is the midpoint of side AC and point Y is the midpoint of side BC. If point R is the midpoint of line segment XC and if point S is the midpoint of line segment YC. What is the area of triangular region RCS? （图麻烦各位画一下）

(1) The area of triangular region ABX is 32.
(2) The length of one of the altitudes of triangle ABC is 8.

我选的是E，也就是两个加一起也推不出来。而正确答案是A，认为第一个自己就充分。

我实在不明白给ABX的面积有啥用？！

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On a certain transatlantic crossing, 20 percent of a ship's passengers held round-trip tickets and also took their cars abroad the ship. If 60 percent of the passengers with round-trip tickets did not take their cars abroad the ship, what percent of the ship's passengers held round-trip tickets?

A. 33又1/3 %
B. 40%
C. 50%
D. 60%
E. 66又2/3%

对这种整体到部分的比例题，我做一道错一道。。拜托各位给点技巧，好吗？

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If M is the least common multiple of 90, 196, and 300. Which of the following is NOT a factor of M?

A. 600
B. 700
C. 900
D. 2,100
E. 4,900

这道题的思路应该怎么想？

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数据充分题（选项省略）

If x and y are positive integers and r is the remainder when 3^(4x + 2) + y is divided by 10, what is the value of r?
（注： 3^(4x + 2)是指3的（4x + 2）次方）

(1) x = 25
(2) y = 1

我选的是C，正确答案是B。
这道题的思路是怎样的？