Find all School-related info fast with the new School-Specific MBA Forum

 It is currently 18 May 2013, 20:06

# Circular gears P and Q start rotating at the same time at

Author Message
TAGS:
Director
Status: Preparing for the 4th time -:(
Joined: 25 Jun 2011
Posts: 558
Location: United Kingdom
GMAT Date: 06-22-2012
GPA: 2.9
WE: Information Technology (Consulting)
Followers: 8

Kudos [?]: 61 [0], given: 212

Circular gears P and Q start rotating at the same time at [#permalink]  20 Mar 2012, 09:04
00:00

Question Stats:

83% (01:59) correct 16% (01:18) wrong based on 15 sessions
Circular gears P and Q start rotating at the same time at constant speeds. Gear P makes 10 revolutions per minute and Gear Q makes 40 revolutions per minute. How many seconds after the gears start rotating will gear Q have made exactly 6 more revolutions than gear P ?

A. 6
B. 8
C. 10
D. 12
E. 15
[Reveal] Spoiler: OA

_________________

Best Regards,
E.

MGMAT 1 --> 530
MGMAT 2--> 640
MGMAT 3 ---> 610

GMAT Club team member
Joined: 02 Sep 2009
Posts: 11506
Followers: 1791

Kudos [?]: 9526 [1] , given: 826

Re: Circular Gears [#permalink]  20 Mar 2012, 09:11
1
KUDOS
enigma123 wrote:
Circular gears P & Q start rotating at the same time at constant speeds. Gear P makes 10 revolutions per minute and Gear Q makes 40 revolutions per minute. How many seconds after the gears start rotating will gear Q have made exactly 6 more revolutions than gear P ?

A. 6
B. 8
C. 10
D. 12
E. 15

Aany idea how to solve this?

Note that we are given revolutions per minute and asked about revolutions in seconds. So we should transform per minute to per second.

Gear P makes 10 revolutions per minute --> gear P makes 10/60 revolutions per second;
Gear Q makes 40 revolutions per minute --> gear Q makes 40/60 revolutions per second.

Let t be the time in seconds needed for Q to make exactly 6 more revolutions than gear P --> \frac{10}{60}t+6=\frac{40}{60}t --> t=12.

Hope it's clear.
_________________
Senior Manager
Joined: 23 Oct 2010
Posts: 335
Location: Azerbaijan
Followers: 6

Kudos [?]: 68 [0], given: 67

Re: Circular gears P and Q start rotating at the same time at [#permalink]  01 Jul 2012, 03:55
(40-10)*t=6

t=1/5 mins

(1/5)*60=12 sec
_________________

Happy are those who dream dreams and are ready to pay the price to make them come true

Intern
Joined: 11 Apr 2012
Posts: 6
Followers: 0

Kudos [?]: 1 [0], given: 4

Re: Circular gears P and Q start rotating at the same time at [#permalink]  18 Apr 2013, 09:28
I used the following approach:

Circular P has 10rpm >> Therefore one rotation will be every 10/60 seconds

Circular Q has 40rpm >> Therefore one rotation will be every 40/60 seconds

Now we are asked after how many seconds will circular Q have made 6 more rotations than Circular P. Translating into algebra

4/6S-1/6(S)=6

Solve for S (where s is number of seconds)
S=12
Re: Circular gears P and Q start rotating at the same time at   [#permalink] 18 Apr 2013, 09:28
Similar topics Replies Last post
Similar
Topics:
A circular rim 28 inches in diameter rotates the same number 8 29 Jul 2003, 15:32
A circular rim 28 inches in diameter rotates the same number 2 15 Apr 2006, 20:59
A circular rim 28 inches in diameter rotates the same number 4 07 Nov 2006, 08:29
7 Circular Gears P and Q start rotating at the same time at 30 07 Nov 2005, 18:49
2 Circular gears P and Q start 3 08 Dec 2008, 13:13
Display posts from previous: Sort by