GMAT Question of the Day - Daily to your Mailbox; hard ones only

 It is currently 15 Aug 2018, 09:45

### GMAT Club Daily Prep

#### Thank you for using the timer - this advanced tool can estimate your performance and suggest more practice questions. We have subscribed you to Daily Prep Questions via email.

Customized
for You

we will pick new questions that match your level based on your Timer History

Track

every week, we’ll send you an estimated GMAT score based on your performance

Practice
Pays

we will pick new questions that match your level based on your Timer History

# Sequence

Author Message
Manager
Joined: 29 May 2008
Posts: 110

### Show Tags

19 Jun 2009, 07:16
00:00

Difficulty:

(N/A)

Question Stats:

0% (00:00) correct 0% (00:00) wrong based on 0 sessions

### HideShow timer Statistics

A certain sequence starts with the number 4. Every sub-sequent term is twice the number 4. Every sub-sequent term is twice the previous one, What is the units digit of the 79 term?

A)8
B)6
C)4
D)2
E)0

I tried to applied the following method

4 firstterm
8 second
16 third
32 fourth
64 fifth
...8 sixth
...6 seventh
...2 eighth
...4 ninth
...8 Tenth

So the Tenth term has a unit digit of 8 I thought that the 80th term would also have a term of 8 so one term less would have 4 the unit-s digit but the answer is D) so what am I doing wrong?

--== Message from GMAT Club Team ==--

This is not a quality discussion. It has been retired.

If you would like to discuss this question please re-post it in the respective forum. Thank you!

To review the GMAT Club's Forums Posting Guidelines, please follow these links: Quantitative | Verbal Please note - we may remove posts that do not follow our posting guidelines. Thank you.
Director
Joined: 03 Jun 2009
Posts: 755
Location: New Delhi
WE 1: 5.5 yrs in IT

### Show Tags

19 Jun 2009, 08:06
TheRob wrote:
I tried to applied the following method

4 firstterm
8 second
16 third
32 fourth
64 fifth
...8 sixth
...6 seventh
...2 eighth
...4 ninth
...8 Tenth

I think you are going in the right direction. Last digit of 1st 4 numbers would be 4 (try 4*8*16*32).
Similarly, for product of each subsequent set of 4 numbers, last digit would be 4.

So, this means we have to find last digit of
..4*..4*..4.....(19 times)* ..4 * ..8 *...6

Now, for each 4*4*4, last digit would be 4
This reduces to
..4*..4*..4...(6 times) *4 *..4 * ..8 *...6
=(..4*..4) *..4 *..4 * ..8 *...6
=..4*..4 * ..8 *...6
=..8

So, last digit should be 8, answer A
_________________
Manager
Joined: 29 May 2008
Posts: 110

### Show Tags

19 Jun 2009, 09:29
It sounds logical however de right answer is D

I will ask the teacher tomorrow and I let you know crapp I almost had the whole homework right
Director
Joined: 03 Jun 2009
Posts: 755
Location: New Delhi
WE 1: 5.5 yrs in IT

### Show Tags

21 Jun 2009, 22:11
TheRob wrote:
It sounds logical however de right answer is D

I will ask the teacher tomorrow and I let you know crapp I almost had the whole homework right

Oops, I got it. I read the question incorrectly, and calculated the last digit of the product of the whole series. I'm sure my GMAT score would be reduced due to this habit of mine.

Anyway, the series would be 4, 8, 16, 32, ..4, ..8, ..6, ..2 and so on
That means, the last digit of every 4th term is 2.

The last digit for 79th term would be 6 and 80th term would be 2
_________________
Director
Joined: 03 Jun 2009
Posts: 755
Location: New Delhi
WE 1: 5.5 yrs in IT

### Show Tags

21 Jun 2009, 22:18
Btw, using my scientific calculator just calculated the value of 79th term of the geometric series, using the formula
nth term = ar^(n-1)

79th term = 1208925819614629174706176
so last digit is 6 only
_________________
Manager
Joined: 29 May 2008
Posts: 110

### Show Tags

22 Jun 2009, 07:19
I finally found out how to solve this one

Well you can see the pattern

4 first
8 second
16 third
32 fourth

4 fifth

Well you divide 79 / 4

76 / 4 = 19 so in the secuence you will Complete the fourth numbers

and you have a remainder of three so you count

first is 4, second is 8 and the third is 6

this was fun though
Intern
Joined: 04 Mar 2008
Posts: 41

### Show Tags

22 Jun 2009, 16:12

--== Message from GMAT Club Team ==--

This is not a quality discussion. It has been retired.

If you would like to discuss this question please re-post it in the respective forum. Thank you!

To review the GMAT Club's Forums Posting Guidelines, please follow these links: Quantitative | Verbal Please note - we may remove posts that do not follow our posting guidelines. Thank you.
Re: Sequence &nbs [#permalink] 22 Jun 2009, 16:12
Display posts from previous: Sort by

# Events & Promotions

 Powered by phpBB © phpBB Group | Emoji artwork provided by EmojiOne Kindly note that the GMAT® test is a registered trademark of the Graduate Management Admission Council®, and this site has neither been reviewed nor endorsed by GMAC®.