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shashank.ism: Legendary Member; Posts: 1022; Joined: Mon Jul 20, 2009 11:49 pmFollowed by: 2 members; Location: Gandhinagar; Thanked: 41 times

by shashank.ism » Wed Feb 10, 2010 7:01 am

A watch ticks 90 times in 95 secs. And another watch ticks 315 times in 323 secs. If they start together, how many times will they tick together in first hour?

a) 101
b) 100
c) 99
d) 102
e) 90

Ans A

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Source: Beat The GMAT — Problem Solving | Wed Feb 10, 2010 7:01 am

money9111: Legendary Member; Posts: 2109; Joined: Sun Apr 19, 2009 10:25 pmFollowed by: 79 members; Location: New Jersey; Thanked: 109 times; GMAT Score:640

by money9111 » Wed Feb 10, 2010 8:29 am

can you explain this? i did lost of multiplying... dividing of fractions... getting confused... improper fractions... D. all of the above and had no such luck

Thanks

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shashank.ism: Legendary Member; Posts: 1022; Joined: Mon Jul 20, 2009 11:49 pmFollowed by: 2 members; Location: Gandhinagar; Thanked: 41 times

by shashank.ism » Wed Feb 10, 2010 8:45 am

money9111 wrote:can you explain this? i did lost of multiplying... dividing of fractions... getting confused... improper fractions... D. all of the above and had no such luck

Thanks

I am providing solution of this problem for reference. But please try it once yourself and then check out for solution.
Well a certain hint is try LCM

[spoiler]First watch ticks after 95/90 seconds.
second watch after 323/315 seconds.
LCM = 19*5*17/45 seconds.
No. of times they will tick in first 3600 seconds = 3600/(19*5*17)/45
Once they have ticked together in the begining , 50 in 1 hour they tick 100+1 = 101 times[/spoiler]

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money9111: Legendary Member; Posts: 2109; Joined: Sun Apr 19, 2009 10:25 pmFollowed by: 79 members; Location: New Jersey; Thanked: 109 times; GMAT Score:640

by money9111 » Wed Feb 10, 2010 12:44 pm

how is 19*5*17 the LCM? I went down to Prime Factors, but I had 2,3,3,5,7

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ldoolitt: Master | Next Rank: 500 Posts; Posts: 184; Joined: Sat Apr 14, 2007 9:23 am; Location: Madison, WI; Thanked: 17 times

by ldoolitt » Wed Feb 10, 2010 1:27 pm

money9111 wrote:how is 19*5*17 the LCM? I went down to Prime Factors, but I had 2,3,3,5,7

Factoring the numerator only...

95 factors to (5, 19)
323 factors to (17, 19)

So the LCM is 5*17*19

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money9111: Legendary Member; Posts: 2109; Joined: Sun Apr 19, 2009 10:25 pmFollowed by: 79 members; Location: New Jersey; Thanked: 109 times; GMAT Score:640

by money9111 » Wed Feb 10, 2010 7:40 pm

aaaahhh i see... i flubbed up the fractions... had it the other way around... ok thanks I get it

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shashank.ism: Legendary Member; Posts: 1022; Joined: Mon Jul 20, 2009 11:49 pmFollowed by: 2 members; Location: Gandhinagar; Thanked: 41 times

by shashank.ism » Wed Feb 10, 2010 10:55 pm

money9111 wrote:how is 19*5*17 the LCM? I went down to Prime Factors, but I had 2,3,3,5,7

It is quite simple money1119 ...
95 = 19 x 5
323 = 17 x 19
so LCM = 17 x 19 x5

then what is the problem in this.. ?????????? I have provided you with whole solution...

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money9111: Legendary Member; Posts: 2109; Joined: Sun Apr 19, 2009 10:25 pmFollowed by: 79 members; Location: New Jersey; Thanked: 109 times; GMAT Score:640

by money9111 » Fri Feb 12, 2010 6:45 pm

no when you said numerator you threw me off because my numerator and denominators were the opposite of yours... so i understand it now..you DID explain it correctly, but as I was looking at what I wrote down... i noticed i messed up the initial set up

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