Check GMAT Club Decision Tracker for the Latest School Decision Releases https://gmatclub.com/AppTrack

 It is currently 27 May 2017, 19:14

### 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 June
Open Detailed Calendar

# A sea beacon consists of two rotating bulbs emitting green

Author Message
SVP
Joined: 03 Feb 2003
Posts: 1604
Followers: 9

Kudos [?]: 268 [0], given: 0

A sea beacon consists of two rotating bulbs emitting green [#permalink]

### Show Tags

10 Aug 2003, 00:17
This topic is locked. If you want to discuss this question please re-post it in the respective forum.

A sea beacon consists of two rotating bulbs emitting green and white rays. At midnight, the bulbs are directly opposite and start to rotate clockwise. The green bulb makes 8 rotations per hour; the white 7 rph. At what time will the rays coincide with each other?
Intern
Joined: 01 Aug 2003
Posts: 48
Location: Mumbai
Followers: 1

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

### Show Tags

10 Aug 2003, 19:12
Solving the problem intuitively, the green bulb gains 1 rotation every 1 hour as in (8-7) rotations. now this means that every one hour, the green bulb gains 360degrees over the white one. now, the initial difference between them is 180degrees which the green bulb has to make up. the rate of gain being 360degrees every hour, it would take 1/2 hours to cover the required 180degrees. to check, in 1/2 hours, the green bulb moves 4 rotations while the white one moves 3.5, after which they both should coincide having started 1/2 a rotation apart.
Intern
Joined: 01 Aug 2003
Posts: 48
Location: Mumbai
Followers: 1

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

### Show Tags

10 Aug 2003, 19:15
or another way to solve is that you know that after every hour, the 2 bulbs should return to their original positions as they each complete a whole number of rotations (8 and 7 respectively) and this is true because 8 and 7 have no common factors. the common factors thing is important as say if you had 4 and 6 instead, then after 1/2 hour, the 4 one would complete 2 rotations and the 6 one would complete 3, hence both returning to their original positions. now since the earliest both return to their respective original positions is an hour, they should cover half of the respective decisions and hence be opposite each other in 1/2 an hour.
10 Aug 2003, 19:15
Display posts from previous: Sort by