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# What is the sum of the numbers common to the two series

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Senior Manager
Joined: 30 Aug 2003
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What is the sum of the numbers common to the two series  [#permalink]  13 Mar 2004, 15:25
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What is the sum of the numbers common to the two series
3,6,9,12,....100 terms & 4,8,12,....80 terms?

7800
3640
3900
5799
None of the above
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Pls include reasoning along with all answer posts.
****GMAT Loco****
Este examen me conduce jodiendo loco

Senior Manager
Joined: 05 May 2003
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3900

If we combine the series we get a new series with 12,24....300
Then, Sn = 1/2 n (a1+an) {n-count ; a1 - first number, an -last number}
As 25 times 12 is 300 I took n as 25.
1/2(25)*(12+300)
Senior Manager
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Posts: 330
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I'm &
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Pls include reasoning along with all answer posts.
****GMAT Loco****
Este examen me conduce jodiendo loco

Senior Manager
Joined: 23 Sep 2003
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Location: US
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3900.

Take the first series. every 4th number is the same as every third number in the second series.

For the first series, 100/4 = 25.
Second series, 80/3 = ~26.6

So they'll have 25 numbers in common. All you have to do is find the sum of these 25 numbers using the first series.

Sum = (2a + (n-1)d) * n/2
a = 12; n = 25; d = 12.

Solve to get 3900.
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