When exposed to sunlight, the # of bacteria in a culture decreases exponentially at the rate of 10% per hour. What is the best approximation for the number of hours required for the initial # of bacteria to decrease by 50%.

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- Jun 6th 2008, 08:38 PMcityismineexponential growth/decay word problem
When exposed to sunlight, the # of bacteria in a culture decreases exponentially at the rate of 10% per hour. What is the best approximation for the number of hours required for the initial # of bacteria to decrease by 50%.

- Jun 6th 2008, 09:54 PMearboth
Let A(0) denote the initial amount of bacteria and A(t) the amount of bacteria after t hours of exposion to sun light.

After 1 h there remains 90% of the bacteria. After t hours remains $\displaystyle 0.9^t$ of the bacteria.

You know that $\displaystyle \frac{A(t)}{A(0)}=\frac12$

Use the equation

$\displaystyle A(t) = A(0) \cdot (0.9)^t$

plug in the values you know and solve for t.

I've got t is roughly 6h 35 min - Jun 6th 2008, 09:54 PMTKHunny
Learning to translate is the point, here.

"# of bacteria in a culture"

Define this. B(t) = # of bacteria in a culture - at time t in hours.

"decreases exponentially"

A clue about $\displaystyle B(t) = ae^{bt}$

"decreases exponentially at the rate of 10% per hour."

A clue about the parameter: $\displaystyle B(1) = ae^{b} = 0.90*ae^{0} = B(0)$

"number of hours required for the initial # of bacteria to decrease by 50%."

B(x) = $\displaystyle 0.90^{x} = 0.50$ -- Solve for x.