At first time all bells toll together at 120sec (i.e, 2 minutes).
But not only at 2 mins they toll together at multiples of 2mins they toll together. That means at 2min, 4min, 6min...........30min, 32,34,..........That is they continue the same.
But they have asked to find tolling only upto 30mins. So now if you divide 30min by 2 you will get 15.
(Thu, Aug 11, 2011 12:36:51 AM)
Can you please tell me how to calculate LCM of above series no. ? Please tell me.
(Sat, Aug 13, 2011 02:37:08 PM)
nw select d max count for 2 3 5
for 2. Its in 8. I.e. 2*2*2.
Then 5 n 3 are only once in any combination.
(Sun, Aug 14, 2011 12:53:56 AM)
Thank you Sravan I got it.
(Sat, Aug 27, 2011 11:17:03 AM)
Thank u deepak...
(Mon, Aug 29, 2011 06:52:09 AM)
Excellent explanation sravanreddy
(Wed, Sep 28, 2011 05:39:56 PM)
But why have counted till 120. ?
(Fri, Sep 30, 2011 09:34:23 PM)
Thanks Sravanreddypailla. Its truly good.
(Sat, Nov 5, 2011 09:55:06 AM)
Why 1 is added?
(Sat, Nov 12, 2011 11:17:59 PM)
Really good explnation.
(Thu, Nov 24, 2011 09:19:03 PM)
@Sravanreddypailla, excellent dude...!!!
(Fri, Dec 2, 2011 08:42:04 AM)
I didn't understand why did we add 1?
(Thu, Dec 8, 2011 12:17:10 PM)
I also didn't understand why did we add '1'? please tell me.
Balaraju Hari said:
(Sun, Dec 18, 2011 04:10:39 PM)
Its simple, according to the shortcut formula everyone understood why the tolling together takes place at every 120sec(2min) period from the l.c.m concept.
But why 1 is added means, it is clearly mentioned in the question that the 6 bells starts tolling together at the starting stage(first tolling is done by together) and tolls at the intervals 2, 4, 6 ,8, 10, 12. so, before tolling together at 120sec(2min), they already tolls together at the 1st(zeroeth sec i.e starting) itself. so that 1 is added.
(Wed, Dec 28, 2011 09:28:10 PM)
Thanks sundar. Amazing explanation!
(Fri, Dec 30, 2011 08:59:07 PM)
Thanks to both Sravan and Sundar for your explanation.
Fozia Sheikh said:
(Wed, Jan 11, 2012 02:34:01 AM)
Four bells toll after intervals of 8, 9, 12and 15minutes, respectively. If they toll together at 3pm. When will they toll together next?
(Wed, Jan 11, 2012 12:51:55 PM)
(Thu, Jan 12, 2012 08:53:55 AM)
Why 30 divided by 2 and 1 added?
(Fri, Feb 10, 2012 11:20:03 PM)
Please present answer to fozia sheihk.
(Sat, Jun 2, 2012 02:29:36 PM)
120seconds is 2 min so divided by 2.
(Fri, Jun 15, 2012 04:38:03 PM)
I guess the answer should be 9pm, since the LCM of 8, 9, 12, 15 is 360. i.e 360 minutes i.e. after 6 hours.
(Wed, Jul 4, 2012 08:40:12 PM)
One is add because it is clearly given in the question that.
"Six bells commence tolling together" that means all bells rang once than after that rang in intervals of 2 4 6 8 10 and 12.
Superb, it is indispensable for general competition.
(Thu, Sep 19, 2013 02:55:10 PM)
Why at last 1 has been added?
Mr Perfect said:
(Fri, Oct 18, 2013 03:07:13 PM)
Since the bells start tolling together, the first toll also needs to be counted. Therefore we need to add 1.
(Fri, Oct 18, 2013 03:37:31 PM)
It's asked in the question that Six bells commence tolling together and toll at intervals of 2, 4, 6, 8 10 and 12 seconds respectively. In 30 minutes, how many times do they toll together?
Here as per the question the 1st bell can toll 15 times in 30 min i.e (30/2), 2nd bell can toll 7.5 times in 30 min i.e 30/4, 3rd bell 5 times, 4th bell 3.7 times, 5th bell 3 times and 6th one 2.5 times in 30 min...together they tolled 36.7 times in 30 min..
Why this approach is wrong?
Lal Chand said:
(Sun, Nov 24, 2013 10:47:37 PM)
Why 30 is divide by 2, please explain?
(Wed, Jan 29, 2014 08:04:05 PM)
Well, I will break it into English but no maths.
Generally all the bells rang (toll) at its first encounter, that means all 6 bells started its journey by joining all their hands at once, i.e., at 0th sec<< remember this started journey>>.
Then they lost their hands and joined again at 120th second, in the mean-while they are tolling individually and not combinedly.
1. It is nothing but at 14th 19th. 25th 38th 57th 69th. Until 119th sec they didn't rang combinedly then at *120th* sec they combined their hands and rung. This is the least number that those 6 bells have waited for their combined toll (nothing but LCM in maths criteria), shortcut used.
2. Then they again rung combinedly after *120*+120th=240th sec+. 120+120+..until 1800.
3. They had given us 30min =1800 sec) , so if we divide that 1800 sec by 120sec (intervals) 1800/120=15 we will get.
Actually this (15) should be the answer if all the bells did not start combinedly, but our bad luck, they mentioned in the question that ""Six bells commence tolling together. "" that means they have started its journey itself by ringing combinedly.
4. The extra +1 is nothing but this started journey.
Now go through what @Saravan and all others said you will get more detailed idea.
Hope this understands. Thank you.
(Mon, Feb 24, 2014 11:15:33 AM)
Why we find the L.C.M.?
I can't understand the L.C.M. method. What is the difference between L.C.M and H.C.M. Why didn't use the H.C.M. method in here. Please anyone can explain me.
(Thu, May 15, 2014 05:56:58 PM)
I am unable to understand why we are using lcm method for this problem. Can anyone explain?
(Sat, Jul 19, 2014 02:41:30 AM)
Why not using HCM:
Suppose 3 bells are tolling at 2, 4, 6 sec,
HCM is 2, so now think, is it possible that they will toll at every 2 sec. NO.
We will have to find a common number (multiple) which is occurring to every bell and that is called LCM.