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# How many prime numbers between 1 and 100 are factors of

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Senior Manager
Joined: 14 Jul 2006
Posts: 281

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How many prime numbers between 1 and 100 are factors of [#permalink]

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25 Aug 2006, 00:23
This topic is locked. If you want to discuss this question please re-post it in the respective forum.

How many prime numbers between 1 and 100 are factors of 7,150 ?

I got 3 -->2,5, 143 why is the OA 4? double count of 5?

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Senior Manager
Joined: 14 Jul 2006
Posts: 281

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25 Aug 2006, 01:03
Thanks VP another careless mistake

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Director
Joined: 28 Dec 2005
Posts: 748

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26 Aug 2006, 10:16
7150 = 2x5x5x11x13

prime factors are 2,5,11,13 = 4

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Current Student
Joined: 29 Jan 2005
Posts: 5201

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26 Aug 2006, 10:37
question should probably read "distinct prime number factors" for clarity. No double counting allowed

2*5^2*11*13

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VP
Joined: 28 Mar 2006
Posts: 1367

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26 Aug 2006, 16:36
BG wrote:
143=11x13

A simple test for 11 divisibility is add alternate #s and subtarct from the other set.

For eg

143

Add 1 and 3 at odd positions

and subtract the sum of the numbers at even positions ie., 4 in this case

(1+3) - 4 = 0

||ly test for any # 1232
1+3 = 4
2+2=4

4-4 =0 so divisible by 11

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26 Aug 2006, 16:36
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# How many prime numbers between 1 and 100 are factors of

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