| P75,110题 A certain clock marks every hour by striking a number of times equal to the hour, and the time required for a stroke is exactly equal to the time interval between strokes . At 6:00 the time lapse between the begining og the first stroke and the end of the last stroke is 22 seconds. At 12:00, how many seconds elapse between the begining of the first stroke and the end of the last stroke? (A) 72, (B) 50, (C) 48, (D) 46, (E) 44. P79, 132题 A company that ships boxes to a total of 12 distribution centers uses color coding to identify each center. If either a single color or a pair of two different colors is chosen to represent each center and if each center is uniquely represented by that choice of one or two colors, what is the minium number of colors needed for the coding?(Assume that the order of the colors in a pair does not matter.) (A) 4, (B) 5, (C) 6, (D) 12, (E) 24. 以上两道题，我实在没有读懂题意，看了后面的解释也懵懵懂懂的，希望大家给我讲解一下. P78, 131题 In the sequence X0,X1, X2.....,Xn, each term from X1 to Xk is 3 greater than the previous term, and each term from Xk+1 to Xn is less than the previous term, where n and k are positive integers and k<n. If X0=Xn=0 and if Xn=15, what is the value of n? (A) 5, (B) 6, (C) 9, (D) 10, (E) 15. 老师，这道题目考的是等差数列的知识点，但是由题可知， X1到Xk是一个等差为3的递增数列，而Xk+1 到Xn 是一个等差为-3的递减数列。 根据绿皮OG 后面的分析，如下： Since X0=0 and each term from X1 to Xk is 3 greater than the precious term, then Xk=0+(k)(3). Since Xk=15, then 15=3k and k=5. Since each term, then Xn=Xk-(n-k)(3). Subsituting the known values for Xk, Xn, and k gives 0=15-(n-5)(3), from which it follows that 3n=30 and n=10. 我的问题是，从题目，我无法判断出Xk与Xk+1是什么关系，属于哪种数列，而上面OG解析红色部分，却用Xk代入方程解题，我对于这个方法，不同意。 但是，如果用Xk+1代入解题则无法得出N的值，烦请告诉我，到底我哪里想的不对. 谢谢大家的帮助！ |
| P75,110题 |
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