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How many positive integers less than 1000 are multiples of 5

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How many positive integers less than 1000 are multiples of 5 [#permalink] New post 14 Apr 2007, 22:34
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A
B
C
D
E

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Question Stats:

50% (01:09) correct 50% (00:01) wrong based on 2 sessions
How many positive integers less than 1000 are multiples of 5 but NOT of 4 or 7?

(A) 114 (B) 121 (C) 122 (D) 129 (E) 136
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Re: PS: Multiples [#permalink] New post 15 Apr 2007, 00:42
kevincan wrote:
How many positive integers less than 1000 are multiples of 5 but NOT of 4 or 7?

(A) 114 (B) 121 (C) 122 (D) 129 (E) 136


There are 1000/5=200 multiples of 5 . now we have to find the number of multiples of 5 and 4 that is 20 between 0 to 1000 and 5 and 7 that is 35 between 0 to 1000.

multiples of 20=1000/20=50 multiples. and multiples of 35= 1000/35 =28.5 so 28 multiples of 35.

So multiples of 5 between o to 1000 not including multiples of 20 and 35 are equal to 200-78 = 122 multiples.
So the answere is C.

Javed.

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 [#permalink] New post 15 Apr 2007, 05:48
It's 129 (D)

There are 199 multiples of 5 that are less than 1000.

Now we have to exclude the multiples of 4 and 7 from these 199 numbers.

Let's first find out the numbers below 1000 that are multiples of 5 as well as 4. (that is, multiples of 20). There are 49 such numbers.

Ditto for 5 as well as 7. There are 28 such numbers.

Subtracting these numbers from 199, we get, 199-49-28=122.

However, there will be some numbers which are multiples of both 4 and 7 which have been subtracted twice in the above calculation.

There are 7 such numbers (multiples of 20 as well as 35, that is multiples of 140).

Thus the required number is 122+7 = 129.
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 [#permalink] New post 15 Apr 2007, 05:49
Javed's method seems to be straight forward and correct.

Although in a counting problem such as this, B is tempting because it's very close to C and it might make you think that you forgot to subtract 1 from somewhere...

What is the OA?
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 [#permalink] New post 15 Apr 2007, 09:08
rakesh.id wrote:
It's 129 (D)

There are 199 multiples of 5 that are less than 1000.

Now we have to exclude the multiples of 4 and 7 from these 199 numbers.

Let's first find out the numbers below 1000 that are multiples of 5 as well as 4. (that is, multiples of 20). There are 49 such numbers.

Ditto for 5 as well as 7. There are 28 such numbers.

Subtracting these numbers from 199, we get, 199-49-28=122.

However, there will be some numbers which are multiples of both 4 and 7 which have been subtracted twice in the above calculation.

There are 7 such numbers (multiples of 20 as well as 35, that is multiples of 140).

Thus the required number is 122+7 = 129.


I agree with your explanation. Answere is 129.

Javed

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 [#permalink] New post 15 Apr 2007, 11:12
multiples of 5: 5(1,2,3,4,5......, 199), 199 in total < 1000
multiples of 4 and 5: 20(1,2,3,4,5,....,49) 49 in total < 1000
multiples of 5 and 7: 35(1,2,3,4,5,....,28) 28 in total < 1000

199 - 49 - 28 = 122

The double counts include multiples of 20,35 (2*2*5, 7*5)
70,140,350,700,770,910,980

122 + 7(doubles) =129
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 [#permalink] New post 15 Apr 2007, 12:51
alfyG wrote:
multiples of 5: 5(1,2,3,4,5......, 199), 199 in total < 1000
multiples of 4 and 5: 20(1,2,3,4,5,....,49) 49 in total < 1000
multiples of 5 and 7: 35(1,2,3,4,5,....,28) 28 in total < 1000

199 - 49 - 28 = 122

The double counts include multiples of 20,35 (2*2*5, 7*5)
70,140,350,700,770,910,980

122 + 7(doubles) =129


I think the double counts given by you are incorrect. 70, for example, is not a multiple of 4 and 5.

The double counts are: 140, 280, 420, 560, 700, 840, 980
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Re: PS: Multiples [#permalink] New post 15 Apr 2007, 14:14
kevincan wrote:
How many positive integers less than 1000 are multiples of 5 but NOT of 4 or 7?

(A) 114 (B) 121 (C) 122 (D) 129 (E) 136


D. I thought of putting my explanation but rakesh.id's is the better one
Re: PS: Multiples   [#permalink] 15 Apr 2007, 14:14
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