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At 15:00 there were 20 students

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by NandishSS » Mon Sep 11, 2017 7:04 am
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

OA: E
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by DavidG@VeritasPrep » Mon Sep 11, 2017 7:36 am
NandishSS 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

OA: E
The number of 3 minute intervals between 15:03 and 15:44 will be the number of multiples of 3 between 3 and 42 inclusive. (42 is the greatest multiple of 3 that is less than 44.) There are 14 of those, so 14*3 =42 students enter the lab.

Between 15:10 and 15:44, there were 4 intervals of 10 minutes, so 4*8 = 32, students leave the lab.

The net change is 42 - 32 = 10. We started with 20 students, so we'll have 20 + 10 = 30 at 15:44. The answer is E
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