Nice Question...

[cramya]

a, b, and c are positive integers. If a, b, and c are assembled into the six-digit number abcabc

Which one of the following must be a factor of abcabc?

(A) 16
(B) 13
(C) 5
(D) 3
(E) none of the above

[ssmiles08]

abcabc = a*100000 + b*10000 + c*1000 + a*100 + b*10 + c

1000(a*100 + b*10 + c) + (a*100 + b*10 + c)

1001(a*100 + b*10 + c)

prime factors of 1001 are 7, 11, 13

7, and 11 are not options, so I think it is 13.

(B)

[cramya]

Well done, Ssmiles.

OA: B

[ghacker]

The important thing in the question is the order , if you spot this then no need to expand

order is abcabc I can write it as-----abc~abc ( two groups) the group to the left is 1000 times the group to the right

let abc = p

in other words abcabc = 1000p +p = 1001p , so if you know the factors of 1001 you know the answer what are the factor of 1001 ?

we know 11 is a factor ( 11*91) so are 7 and 13 (91=7*13)

Hence the answer is 13

what I see is that when one gets a question similar to this one it should be answerable with in 1 min so there must be a catch to the question
just think why did the question say abcabc ---- because something is there

If you see this the question becomes very easy

[cramya]

If you see this the question becomes very easy

Agree.

Part of GMAT is about spotting patterns(i.e. if one exists). If one is successful in doing so under exam pressure/timing then it becomes really easy and if not then it becomes difficult

Good luck, All!