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At 15:00 there were 20 students in the computer lab. At 15:03 and ever

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At 15:00 there were 20 students in the computer lab. At 15:03 and ever [#permalink]

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New post 10 Aug 2010, 23:23
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Difficulty:

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Question Stats:

63% (02:00) correct 37% (02:11) wrong based on 70 sessions

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At 15:00 there were 20 students in the computer lab. At 15:03 and every three minutes after that, 3 students entered the lab. If at 15:10 and every ten minutes after that 8 students left the lab, how many students were in the computer lab at 15:44 ?

A. 7
B. 14
C. 25
D. 27
E. 30
[Reveal] Spoiler: OA

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Re: At 15:00 there were 20 students in the computer lab. At 15:03 and ever [#permalink]

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New post 10 Aug 2010, 23:44
bibha wrote:
At 15:00 there were 20 students in the computer lab. At 15:03 and every three minutes after that, 3 students entered the lab. If at 15:10 and every ten minutes after that 8 students left the lab, how many students were in the computer lab at 15:44 ?
7
14
25
27
30


Initial no of students + 3 * (1 + No of possible 3 minute intervals between 15:03 and 15:44) -
8 *(1 + No of possible 10 minute intervals between 15:10 and 15:44)

20 + 3*14 -8 * 4 = 30
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Re: At 15:00 there were 20 students in the computer lab. At 15:03 and ever [#permalink]

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New post 11 Aug 2010, 07:07
Could you please explain why you are adding 1???

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Re: At 15:00 there were 20 students in the computer lab. At 15:03 and ever [#permalink]

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New post 11 Aug 2010, 08:32
i used the same method as CRACK700.

the reason you add a 1 is to make sure you add back a subtracted number.

for example:
how many numbers are between 6 and 10, inclusive? if we simply say 10 - 6 = 4, we would not be taking into consideration the number 6. with 4, we only have 10, 9, 8, and 7; only 4 numbers. it should be 10, 9, 8, 7, and 6; 5 numbers. so we add the one to add back the 6.
10 - 6 = 4 + 1 = 5 numbers between 10 and 6.

this method is used in this problem. is this clear by chance? hopefully it is.
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Re: At 15:00 there were 20 students in the computer lab. At 15:03 and ever [#permalink]

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New post 11 Aug 2010, 14:20
3 students are added every three minutes till 15:44 and every 10 minutes 8 people are subtracted. This means we need to find out max multiple of 3 and 10 to get the total number of students added and subtracted respectively
20 + 14 * 3 - 4*8
= 30 answer

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Re: At 15:00 there were 20 students in the computer lab. At 15:03 and ever [#permalink]

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New post 23 Sep 2017, 23:16
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Re: At 15:00 there were 20 students in the computer lab. At 15:03 and ever   [#permalink] 23 Sep 2017, 23:16
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