How do you solve this.
A blue light flashes four times per minute and an orange light flashes five times per minute at regular intervals. If both the lights start flashing at the same time, how many times do they flash together per hour?
Answer Choices
A. 42
B. 120
C. 60
D. 36
E. 80
Thanks,StrawberryCow
Number System - many times do they flash together per hour
This topic has expert replies
-
- Junior | Next Rank: 30 Posts
- Posts: 20
- Joined: Tue Oct 29, 2013 8:38 pm
- Followed by:2 members
- Uva@90
- Master | Next Rank: 500 Posts
- Posts: 490
- Joined: Thu Jul 04, 2013 7:30 am
- Location: Chennai, India
- Thanked: 83 times
- Followed by:5 members
Hi,StrawberryCow wrote:How do you solve this.
A blue light flashes four times per minute and an orange light flashes five times per minute at regular intervals. If both the lights start flashing at the same time, how many times do they flash together per hour?
Answer Choices
A. 42
B. 120
C. 60
D. 36
E. 80
Thanks,StrawberryCow
Is Answer C
This is how I did,
1 hour = 3600 sec.
Blue light flashes once in 15 sec(60/4)
Orange light flashes once in 12 sec(60/5)
Find LCM for 15 and 12 = 60
Now find how many 60's are there in 3600 =3600/60 = 60
Hence Answer is 60
Regards,
Uva.
Known is a drop Unknown is an Ocean
Blue light flash once in 60/4 = 15 sec
Orange light flash once in 60/5 = 12 sec
Blue and orange light will flash together = LCM of 15 and 12 = 60 sec -> 1 min (after every)
So, after an hour blue and orange light will flash together = 60 times..
Orange light flash once in 60/5 = 12 sec
Blue and orange light will flash together = LCM of 15 and 12 = 60 sec -> 1 min (after every)
So, after an hour blue and orange light will flash together = 60 times..
-
- GMAT Instructor
- Posts: 2630
- Joined: Wed Sep 12, 2012 3:32 pm
- Location: East Bay all the way
- Thanked: 625 times
- Followed by:119 members
- GMAT Score:780
One way to do this if you're stuck on the test is to see how many times the lights flash simultaneously in one minute, then to multiply that by 60 to find out how many times they flash simultaneously per hour. (This won't always work, but it happens to work here for reasons I'll detail at the end of the post ... and it can't hurt to learn a practical alternative!)
Say we start our minute at 00:00 (minutes : seconds). The blue light flashes at 00:15, 00:30, 00:45, 1:00. The orange light flashes at 00:12, 00:24, 00:36, 00:48, 1:00. Since the only time they flash together is at 1:00, they only flash together once per minute, giving us 60 times per hour.
We could get in trouble if the lights don't both flash at 1:00 (as they might go into different cycles in the second minute), but here we don't have to worry about that - as soon as we see that the cycle is the same every minute, we're fine. If the problem had some other cycle, we'd just have to determine when the cycle repeats, then generalize that from minutes to hours.
Say we start our minute at 00:00 (minutes : seconds). The blue light flashes at 00:15, 00:30, 00:45, 1:00. The orange light flashes at 00:12, 00:24, 00:36, 00:48, 1:00. Since the only time they flash together is at 1:00, they only flash together once per minute, giving us 60 times per hour.
We could get in trouble if the lights don't both flash at 1:00 (as they might go into different cycles in the second minute), but here we don't have to worry about that - as soon as we see that the cycle is the same every minute, we're fine. If the problem had some other cycle, we'd just have to determine when the cycle repeats, then generalize that from minutes to hours.