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September 22, 2014

September 22, 2014

Posted by **Jake** on Saturday, January 28, 2012 at 9:34pm.

1. The population of the bacteria in a Petri dish increases by a factor of 10 every 10 hour. If there are initally 20 bacteria in the dish, how long will it take before the population increases to 60,000 to the nearest tenth of an hour?

a) 3000.0 h

b) 300.0 h

c) 4.8 h

d) 3.5 h

2. A certain radioactive isotope has a half life of 5 years. If a sample has 120 grams of radioactive material, what equation could be used to determine the amount of material remaining (y) after x years.

a) y=120x(1/2)^5x

b) y=60^5x

c) y=120x5^1/2x

d) y=1/2x120^5x

- Math -
**Reiny**, Sunday, January 29, 2012 at 9:27am1.

doubling time is 10

amount = 20(10)^(x/10)

so ...

60000= 20(10)^(x/10)

3000 = 10^(x/10)

log both sides

log 3000 = log 10^(x/10)

x/10 = log 3000

x = 10log3000 = 34.8 hrs.

proof

at end of 10 hours; 20(10) = 200

at the end of 20 hrs = 200(10) = 2000

at the end of 30 hrs = 2000(10) = 20000

at the end of 40 hrs = 20000(10) = 200000

so the time must have been between 30 and 40 hours

amount = 20(10)^34.8/10)

= 20(10)^3.48

= 60399

none of your choices are correct.

2.

amount = 120(1/2)^(x/5)

the way you typed your choices, none match my answer.

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