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

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

by guerrero » Thu Jun 27, 2013 5:50 pm
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

what logic is tested here ? I am clueless ...:(

OAE
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by srcc25anu » Thu Jun 27, 2013 7:19 pm
At 15:00 there are 20 students

STUDENTS WHO ENTER:
from 15:03 to 15:44, every 3 minutes 3 students are added.
# of 3 min intervals from 15:03 to 15:44, (44-3)+1/3 = 14
No of students who will enter = 14 * 3 = 42

STUDENTS WHO LEAVE:
from 15:10 to 15:44, every 10 minutes 8 students leave the room.
# of 10 min intervals from 15:10 to 15:44, (44-10)+1/10 = 4
No of students who will enter = 4 * 8 = 32

Net student in class at 15:44 = 20 + 42 - 32 = 30

Ans E
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by GMATGuruNY » Thu Jun 27, 2013 8:01 pm
guerrero 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 ?

a)7
b)14
c)25
d)27
e)30

what logic is tested here ? I am clueless ...:(

OAE
At :00, the number of students = 20.

From :00 to :44, every three minutes, 3 students enter.
Since 3 minutes divides into 44 minutes a total of 14 times, the number of students will INCREASE by 3 a total of 14 times:
20 + 14*3 = 62.

From :00 to :44, every ten minutes, 8 students leave.
Since 10 minutes divides into 44 minutes a total of 4 times, the number of students will DECREASE by 8 a total of 4 times:
62 - 8*4 = 30.

Thus, at 15:44, the number of students = 30.

The correct answer is E.
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