[求助] prep test1的一题~~~~

jiajia0804

 If n is a positive integer and r is the remainder when 4+7n is divided by 3, what is the value of r?
1) n+1 is divisible by r
2) n >20

please~~ who can help me to solve the problem?

jiajia0804

自己顶一下！！

大家帮帮忙吧～～

下个月就要考了～～～

bigskings

我帮你在国外网找了答案。别人的东西，我就不翻译了，给你看英文的。

bigskings

I believe filling-in would ease the situation here. Also note that the only values 'r' can take are 0,1&2.
we can take three variables: n, n+7n & r (remainder when n+7n is divided by 3):

n (n+7n) r
1 11 2
2 18 0
3 25 1
4 32 2

Stmt-A:n+1 divisible by r
This is possible in case:1 & 3. But case:1 doesn't always hold true (as can be seen by a parallel case:4), hence the value of r has to be 1. SUFFICIENT.


Stmt-B: It only gives a range of values. INSUFFICIENT.

Answer should be A.

bigskings

I agree with A, but I have a slightly different approach

Stem 7n + 4 leaves a remainder r when divided by 3. So r is 0,1 or 2

Stmt 1
n+1 is divisible by r
We know 7n + 4 = 7(n+1) - 3
From stem 7(n+1) when by 3 gives a remainder r and moreover 7*(n+1) is cleanly divided by r. So r is not 0. It can be 1 or 2 ONLY

We see from Stmt n + 1 is divided by r so r can only be 1 and cant be 2 as it wud fail for even values of n

Stmt 2 offers nothing

bigskings

copy from: http://www.urch.com/forums/

jiajia0804

thank you啊~~~~~

大大大好人！~~`

我今天才做了第二遍prep，呵呵~~

看来我要加快复习进度了！~~~