N:Dure wrote:An antibiotic is added to a culture of bacteria and is found to kill 1/3 of the bacteria in the culture every 30 seconds. If a lab assistant checks the culture after 2 minutes, what fraction of the original bacteria will have been killed?
a) 16/81
b) 8/27
c) 19/27
d) 65/81
e) 80/81
We can plug in a value for the bacteria.
Let bacteria = 81.
After first 30 seconds, 1/3*81 = 27 killed, 81-27 = 54 left.
After next 30 seconds, 1/3*54 = 18 killed, 54-18 = 36 left.
After next 30 seconds, 1/3*36 = 12 killed, 36-12 = 24 left.
After next 30 seconds, 1/3*24 = 8 killed, 24-8 = 16 left.
Since only 16 are left, 81-16 = 65 were killed.
Killed/Total = 65/81.
The correct answer is
D.
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