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# Aptitude - Problems on H.C.F and L.C.M - Discussion

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"Two things are infinite: the universe and human stupidity; and I'm not sure about the universe."
- Albert Einstein
17.

What will be the least number which when doubled will be exactly divisible by 12, 18, 21 and 30 ?

 [A]. 196 [B]. 630 [C]. 1260 [D]. 2520

Explanation:

``` L.C.M. of 12, 18, 21 30                 2 | 12  -  18  -  21  -  30
----------------------------
= 2 x 3 x 2 x 3 x 7 x 5 = 1260.       3 |  6  -   9  -  21  -  15
----------------------------
Required number = (1260 ÷ 2)            |  2  -   3  -   7  -   5

= 630.
```

 Saras said: (Tue, Dec 21, 2010 04:41:53 AM) why we are dividing by 2?? pl tel me and also tel me some easy tricks for h c m and l c m

 Aparna said: (Sun, Dec 26, 2010 06:13:40 AM) pls remember whenever dividend is unknown,it is an LCM question so firstly find the LCM of 12,18,21,30 we get 1260,and this is exactly divisible by 12,18,21,30 as we have asked after doubling the number,it is exactly divisible by the given numbers. so,1260/2 is done,we get 630 and 630*2=1260,which is asked to us

 Priz said: (Mon, Dec 27, 2010 01:51:19 AM) Thanks Aparna, it is really helpful

 Grishma said: (Tue, Jun 14, 2011 11:07:27 PM) How 1260/2 comes?

 Deepak said: (Mon, Jul 18, 2011 11:51:45 AM) 1260/2 come ,because in Q it ask to find least no.(630) which when double to divisible by given no.(12,18,21,30)