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Bunuel
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This is a high quality question.

Posted from my mobile device
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Bunuel
Official Solution:

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

During the 44 minutes between 15:00 and 15:44 \(\frac{42}{3} = 14\) groups, 3 students each, entered the lab and \(\frac{40}{10} = 4\) groups, 8 students each, left the lab. Thus, the number of students in the lab grew by \(14*3 - 8*4 = 10\).

Answer: E


Can someone please explain this in detail. How did we get 42 and 40 in numerators.
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Bunuel
Official Solution:

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

During the 44 minutes between 15:00 and 15:44 \(\frac{42}{3} = 14\) groups, 3 students each, entered the lab and \(\frac{40}{10} = 4\) groups, 8 students each, left the lab. Thus, the number of students in the lab grew by \(14*3 - 8*4 = 10\).

Answer: E


Can someone please explain this in detail. How did we get 42 and 40 in numerators.

First 3 students entered the lab at 15:03. Next:
15:06;
15:12;
...
15:39
Last 3 students entered the lab at 15:42.

As you can see we have multiples of 3 from 3 to 42 inclusive. How many multiples of 3 are there in this range? (42 - 3)/3 + 1 = 14.

First 8 student group left the lat at 15:10.
15:20
15:30
15:40.
4 groups, 8 students each, left the lab.
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I have edited the question and the solution by adding more details to enhance its clarity. I hope it is now easier to understand.
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I did not quite understand the solution. would it not be 3 intervals instead for 10 mins interval? 3:10-20, 3:20-30, 3:30 - 40
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We can also solve this with AP.

Notice the series
15:00-20
15:03-23
15:06-26
.
.
.
15:42-x

common difference is 3, 1st term is 20, no. of terms is 15
so last term=1st term+(no. of term-1)common difference
x=20+14*3=62

We need to find the population at 15:44, the next addition will be 15:45. So, strategically 15:42 population is fine till here.

Now it has been given that at eac interval of 10 minutes 8 students are leaving, starting from 15:10,
so, at 15:10=-8
15:20=-8
15:30=-8
15:40=-8

total student left=-32

the next batch of 8 student will leave at 15:50.

As we have got the 62 as population when 3 are added every 3 minute, now we got 32 as 8 leaving every 10 min from the given time 15:03 and 15:10 respectively.
Therefore at 15:44 there will 62-32=30 students left.
Bunuel
At 15:00, 20 students were in the computer lab. Starting at 15:03, and every three minutes thereafter, 3 students entered the lab. Meanwhile, beginning at 15:10 and every ten minutes thereafter, 8 students exited the lab. How many students were in the computer lab at 15:44?

A. 7
B. 14
C. 25
D. 27
E. 30
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