I need explanation for 1000 PS question

babachal

If n is an integer, which of the following CANNOT be a factor of 3n + 4 ?

(A) 4
(B) 5
(C) 6
(D) 7
(E) 8

llewellyn27 · Posted: Thu Jun 12, 2008 2:07 pm Post subject:

I think the answer is 6

using the eqn 3n+4 (Let n = 1,2,3,4,5,6,7)

By process of elimination the answer is 6

Ex:

3*1 + 4 = 7
3*2 +4 = 10(Divisible by 5)
3*4 + 4 = 16 (Divisible by 4 and 8)

and so on till u get only 6 remaining

TOOK ME 2 MINS

babachal · Posted: Thu Jun 12, 2008 2:53 pm Post subject:

i did not get it, sorry, look

3*4+4=16
3*5+4=19
3*6+4=22
3*7+4=25
3*8+4=28

now 5,7,and 8 can not be factor of the what from formula equal as well as 6, so how did you pick this number?

babachal · Posted: Thu Jun 12, 2008 3:04 pm Post subject:

oh, sorry now i got it,

thank you!

Ian Stewart · Posted: Thu Jun 12, 2008 4:53 pm Post subject:

You might also look at it this way: you can divide 3n+3 by 3 (it's equal to 3*(n+1)), so you definitely cannot divide 3n+4 by 3; you'll always get a remainder of 1, because it's one greater than a multiple of 3. If you can't divide 3n+4 by 3, you definitely can't divide it by 6, or any other multiple of 3.