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01 Jul 2019, 02:16
Kudos
Bookmarks
C
Be sure to select an answer first to save it in the Error Log before revealing the correct answer (OA)!
Difficulty:
45%
(medium)
Question Stats:
64% (01:43) correct
36%
(02:07)
wrong
based on 244
sessions
History
Date
Time
Result
Not Attempted Yet
The population of a certain type of bacterium triples every 10 minutes. If the population of a colony 20 minutes ago was 100, in approximately how many minutes from now will the bacteria population reach 24,000?
A. 20 minutes
B. 25 minutes
C. 30 minutes
D. 35 minutes
E. 40 minutes
A. 20 minutes
B. 25 minutes
C. 30 minutes
D. 35 minutes
E. 40 minutes
Bunuel
Population now is 100 *3 *3 = 900
Population in future = \(\frac{24000}{900}\) = 26\(\frac{2}{3}\) times = approx 27 times = \(3^3\) times
It will take 30 minutes
IMO C
01 Jul 2019, 02:33
Kudos
Bookmarks
Bunuel
Method of counting
At -20 minutes, total = 100
At -10 minutes, total = 300
At 0 minutes, total = 900
At 10 minutes, total = 2700
At 20 minutes, total = 8100
At 30 minutes, total = 24300
Alternate Method: Using GP Series
1st term, a = 100, ratio, r = 3
24000 = 100*r^(n - 1)
--> 3^(n-1) = 240
--> 3^(n-1) = 243 (approx)
--> 3^(n-1) = 3^5
--> n - 1 = 5
--> n = 6
--> -20, -10, 0, 10, 20, 30
IMO Option C
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