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

 It is currently 18 Nov 2018, 19:10

### 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

## Events & Promotions

###### Events & Promotions in November
PrevNext
SuMoTuWeThFrSa
28293031123
45678910
11121314151617
18192021222324
2526272829301
Open Detailed Calendar
• ### How to QUICKLY Solve GMAT Questions - GMAT Club Chat

November 20, 2018

November 20, 2018

09:00 AM PST

10:00 AM PST

The reward for signing up with the registration form and attending the chat is: 6 free examPAL quizzes to practice your new skills after the chat.
• ### The winning strategy for 700+ on the GMAT

November 20, 2018

November 20, 2018

06:00 PM EST

07:00 PM EST

What people who reach the high 700's do differently? We're going to share insights, tips and strategies from data we collected on over 50,000 students who used examPAL.

# The rate of spin of a certain gyroscope doubled every 10 seconds from

 new topic post reply Question banks Downloads My Bookmarks Reviews Important topics
Author Message
TAGS:

### Hide Tags

Manager
Joined: 02 Jun 2015
Posts: 191
Location: Ghana
The rate of spin of a certain gyroscope doubled every 10 seconds from  [#permalink]

### Show Tags

31 Dec 2015, 10:01
1
7
00:00

Difficulty:

45% (medium)

Question Stats:

65% (02:07) correct 35% (02:03) wrong based on 218 sessions

### HideShow timer Statistics

The rate of spin of a certain gyroscope doubled every 10 seconds from the moment a particular stopwatch started. If after a minute and a half the gyroscope reached a speed of 400 meters per second, what was the speed, in meters per second, when the stopwatch was started?

A) 25/3
B) 25/4
C) 25/8
D) 25/16
E) 25/32

_________________

Kindly press kudos if you find my post helpful

CEO
Joined: 11 Sep 2015
Posts: 3122
Re: The rate of spin of a certain gyroscope doubled every 10 seconds from  [#permalink]

### Show Tags

31 Dec 2015, 10:15
2
2
duahsolo wrote:
The rate of spin of a certain gyroscope doubled every 10 seconds from the moment a particular stopwatch started. If after a minute and a half the gyroscope reached a speed of 400 meters per second, what was the speed, in meters per second, when the stopwatch was started?

A) 25/3
B) 25/4
C) 25/8
D) 25/16
E) 25/32

This can be answered within a reasonable time frame by just charting each 10-second period.

- After 90 seconds: 400 meters/second
- After 80 seconds: 200 meters/second
- After 70 seconds: 100 meters/second
- After 60 seconds: 50 meters/second
- After 50 seconds: 25 meters/second
- After 40 seconds: 25/2 meters/second
- After 30 seconds: 25/4 meters/second
- After 20 seconds: 25/8 meters/second
- After 10 seconds: 25/16 meters/second
- START: 25/32 meters/second

For more on this straightforward technique, see our free video - http://www.gmatprepnow.com/module/gmat- ... /video/929

Cheers,
Brent
_________________

Test confidently with gmatprepnow.com

Math Expert
Joined: 02 Aug 2009
Posts: 7037
Re: The rate of spin of a certain gyroscope doubled every 10 seconds from  [#permalink]

### Show Tags

07 Jan 2016, 00:11
4
duahsolo wrote:
The rate of spin of a certain gyroscope doubled every 10 seconds from the moment a particular stopwatch started. If after a minute and a half the gyroscope reached a speed of 400 meters per second, what was the speed, in meters per second, when the stopwatch was started?

A) 25/3
B) 25/4
C) 25/8
D) 25/16
E) 25/32

Hi,
two ways you can do the Qs of this type..

1) working backwards..

here you will have to carry out HOW many steps- 90/10..
can be error prone, but the way would be..
after
90 secs - 400..
80 secs - 200.., keep dividing by 2
70 secs - 100
60 secs - 50
50 secs - 25
40 secs - 25/2
30 secs - 25/4
20 secs - 25/8
10 secs - 25/16..
initial - 25/32..

2) straight

..

an easier and better, and less time consuming method

let the initial speed be x ..
it doubles in 10 secs, or it increases 2 times every 10 secs..
total 10 secs period= 90/10=9..
so final speed after 90 secs= x*2*2...9 times
therefore $$x*2^9=400$$...
or $$x*2^9=400=2^4*25$$...
$$x=25*2^4/2^9=25/2^5=25/32..$$
ans E
_________________

1) Absolute modulus : http://gmatclub.com/forum/absolute-modulus-a-better-understanding-210849.html#p1622372
2)Combination of similar and dissimilar things : http://gmatclub.com/forum/topic215915.html
3) effects of arithmetic operations : https://gmatclub.com/forum/effects-of-arithmetic-operations-on-fractions-269413.html

GMAT online Tutor

Manager
Joined: 09 Dec 2015
Posts: 114
Location: India
Concentration: General Management, Operations
Schools: IIMC (A)
GMAT 1: 700 Q49 V36
GPA: 3.5
WE: Engineering (Consumer Products)
Re: The rate of spin of a certain gyroscope doubled every 10 seconds from  [#permalink]

### Show Tags

10 Jul 2017, 08:03
1
Long method, count backwards....

Shortcut, use Geometric Progression.
Nth term of a GP is, tn = a*r^n (where a is the first term of progression and r is the ratio of second term to first term)
We know, r = 2 (as speed doubles)
tn = 400 (speed at 90 sec)
n= 9 (since there will be 9 intervals of 10 sec in 90 sec)
so, substitute all these values in equation above.
400 = a * 2^9 or, 400 = a* 512
therefore, a = 400/512 = 25/32.

This method looks long but it only takes 10-15 sec to solve.
Non-Human User
Joined: 09 Sep 2013
Posts: 8817
Re: The rate of spin of a certain gyroscope doubled every 10 seconds from  [#permalink]

### Show Tags

17 Sep 2018, 03:03
Hello from the GMAT Club BumpBot!

Thanks to another GMAT Club member, I have just discovered this valuable topic, yet it had no discussion for over a year. I am now bumping it up - doing my job. I think you may find it valuable (esp those replies with Kudos).

Want to see all other topics I dig out? Follow me (click follow button on profile). You will receive a summary of all topics I bump in your profile area as well as via email.
_________________
Re: The rate of spin of a certain gyroscope doubled every 10 seconds from &nbs [#permalink] 17 Sep 2018, 03:03
Display posts from previous: Sort by

# The rate of spin of a certain gyroscope doubled every 10 seconds from

 new topic post reply Question banks Downloads My Bookmarks Reviews Important topics

 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®.