Why use LCM?

This topic has expert replies
Newbie | Next Rank: 10 Posts
Posts: 8
Joined: Sun Jun 21, 2009 8:20 am

Why use LCM?

by NYC_Kid » Sun Jun 21, 2009 11:00 am
Here's the question:

A wire is cut into three equal parts. The resulting segments are then cut into 4, 6 and 8 equal parts respectively. If each of the resulting segments has an integer length, what is the minimum length of the wire?

a) 24
b) 36
c) 48
d) 54
e) 72

I would think (4+6+8) * 3= 54 would be the correct answer.

But you have to take the LCM of 4,6,8 and you would get 24, and then you would multiply 24 *3= 72. Why would you do the LCM? I have a hard time visualizing this.

Senior | Next Rank: 100 Posts
Posts: 40
Joined: Mon Feb 16, 2009 8:50 pm
Thanked: 3 times

by crackthetest » Sun Jun 21, 2009 11:20 am
There are 2 pieces here, first is cutting into 3 equal parts and then splitting those into 4:6:8 ratio, and the questions asks for minimum length, so you'll have to take LCM for those ratios which is 24. The total length of the wire is hence 24*3 = 72.

GMAT/MBA Expert

User avatar
GMAT Instructor
Posts: 2621
Joined: Mon Jun 02, 2008 3:17 am
Location: Montreal
Thanked: 1090 times
Followed by:355 members
GMAT Score:780

by Ian Stewart » Sun Jun 21, 2009 11:21 am
We first divide the length of the wire into three equal parts. Call the length of each part P; we know that P is divisible by 4, 6 and 8. The smallest number which is divisible by 4, 6 and 8 is, by definition, the LCM of 4, 6 and 8, or 24. So the smallest value of P is 24, and since we the total length of wire is 3P, the answer is 72.
For online GMAT math tutoring, or to buy my higher-level Quant books and problem sets, contact me at ianstewartgmat at gmail.com

ianstewartgmat.com

Newbie | Next Rank: 10 Posts
Posts: 8
Joined: Sun Jun 21, 2009 8:20 am

by NYC_Kid » Sun Jun 21, 2009 11:35 am
Thanks guys.

:D

Master | Next Rank: 500 Posts
Posts: 345
Joined: Wed Mar 18, 2009 6:53 pm
Location: Sao Paulo-Brazil
Thanked: 12 times
GMAT Score:660

by shibal » Thu Jul 09, 2009 1:36 pm
i did it in a different way.
if the wire is divided into 3 parts (x/3, x/3 and x/3) and then later divided into 4,6,8 parts (which would be equal to x/12, x/18 and x/24), pick numbers given as answers to see where all three equations will give you integers.... 72 is the only one....