求教prep上的两道余数题+1道概率题

foofie

1. When positive integer n is divided by 3, the remainder is 2, and when posibive integter t is divided by 5, the remainder is 3, what is the remainder when the product nt is divdided by 15?
(1) n-2 is divisible by 5
(2) t is divisible by 3
The answer is C.
由题目得出n-2可以被3整除，t-3可以被5整除，目标是求出n和t被15整除的余数，然后相乘
条件（1），可以知道n-2可以被15整除，所以n被15除，余数是2
条件（2），如何求出t被15除的余数呢？
谢谢

2. Is the integer n odd?
(1) n is divisible by 3
(2) 2n is divisible by twice as many positive integers as n
The answer is B
求解题思路，以下是我的代入法，有点傻。
如果n是2，那就被1和2divisible，2n就是4，被1，2，4divisible
如果n是3，那就被1和3divisible， 2n就是6，被1，2，3，6divisible
如果n是4，那就被1，2，4 divisible，2n就是8，被1，2，4，8 divisible
如果n是5，那就被1，5 divisible， 2n就是10， 被，1，2，5，10 divisible

3. If the drama club and music club are combined, what percent of the comabined membership will be male?
(1) Of the 16 members of the drama club, 15 are male
(2) Of the 20 members of the music club, 20 are male
The answer is E.
为何不是c啊，这个概率不是 = （15+20）/(16+20) ？

Dub23

3. If the drama club and music club are combined, what percent of the comabined membership will be male?
(1) Of the 16 members of the drama club, 15 are male
(2) Of the 20 members of the music club, 20 are male


会不会有重复的人呢

angelazhao222

我也不懂  就答

foofie

在prep详解里找到了第1题分享
【思路】
n=3x+2
t=5y+3
nt =15xy+10y+9x+6
(1) n-2 = 5a => 3x+2-2 = 5a => 3x is divisible by 5 => n=15b+2
(2) t = 3c => 5y is divisible by 3 => t=15d+3

(1)+(2)
nt = 15Z + 6

foofie

Dub23 说得有道理，有这个可能