GMAT Prep - a bit strange Q

[sudaif]

Can Ron or one of the instructors please explain a methodological way to approach this question? i was a bit stumped and running out of time on GMAT prep practice test. The other solutions posted to this question are generally not v helpful. thanks.

If each term in the sum a1 + a2 + a3...+ an is either 7 or 77 and the sum equals 350, which of the following could be n?

A) 38
B) 39
C) 40
D) 41
E) 42

OA is C

[adiagr]

Can Ron or one of the instructors please explain a methodological way to approach this question? i was a bit stumped and running out of time on GMAT prep practice test. The other solutions posted to this question are generally not v helpful. thanks.

If each term in the sum a1 + a2 + a3...+ an is either 7 or 77 and the sum equals 350, which of the following could be n?

A) 38
B) 39
C) 40
D) 41
E) 42

OA is C

Hi Sudaif, Ron's answer may be seen at this link:

sequence-each-term-is-either-7-or-77-t2731.html?hilit=350

Aditya

[rash.patil]

Another way:

Since the sum is 350, it should be a multiple of 10. As we have only 7 and 77 in the series, we can write it as
x * 7 + y * 77 = 350.

77 = 7 * 11
as 7 and 11 are both prime numbers, sum can be 350 when the x + y is a multiple of 10.
check the answers.
Only possible solution is 40.

[RonPurewal]

one of the other posters was kindly helpful enough post a link to one of my previous explanations; please go check it out.
if you have any questions about that explanation, post them on THAT thread, not here.

The other solutions posted to this question are generally not v helpful. thanks.

please do not do this -- do not start a new thread for a problem that already has an existing thread(s) THAT YOU HAVE FOUND on this forum.
if you see a problem that is already posted elsewhere on the forum, and you have questions about that problem, please post your questions on the already existing thread.
we do not want to have a situation in which there are several different threads for each problem.

if you just can't find an existing thread on the problem, then by all means go ahead and post a new thread. (for math problems in particular, this is sometimes understandable -- there are many problems with just equations/inequalities in them, and no words, for which it's extremely difficult to search.)
however, if you have actually found existing threads on the problem (which you clearly have here, as you have referred to "the other explanations"), than please do not create additional threads.

thanks.