Question 148605
 A culture of yeast doubles in size every 20 minutes. The size of the culture is now No. Find its size in 12 hours.
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A(t) = No(2)^(t/20) where t is the number of hours
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A(12) = No(2)^(12*(60/20)) = No*2^36 = appox. 6.87..x 10^10*No
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Sugar is put in a large quantity of water and the mixture is stirred. After 2 minutes, 50% of the sugar has dissolved. How much longer will it take until 90% of the sugar has dissolved? 
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If 50% has dissolved after 2 minutes, 50% remains after two minutes.
You want to know how long till only 10% of No remains.
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EQUATION:
When does No(1/2)^(x/2) = 0.1No
(1/2)^(x/2) = 0.10
Take the log of both sides to get:
(x/2)*log(1/2) = log 0.1
(x/2) = 3.321928...
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Number of minutes = 2*3.321978.. = 6.643856...
This is total time including the 1st two minutes.
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Cheers,
Stan H.