# Thread: Which equation would I use?

1. ## Which equation would I use?

I have some math problems that I'm confused on...

I was given these formulas to work with:

$A(t) = A_{0}(1+r)^t$
$A(t) = A_{0}b^{tk}$
$P(t) = P_{0}e^{rt}$
$A(t) = A_{0}(1\\\pm r/k)^{t*k}$

If 500 fleas triple in their population every 18 hours:
1) How many fleas will there be in 3 days?
I got 40500, but will someone confirm whether this is right?

2) How long will it take for there to be 1200 fleas?
I got 14.34 hours, but again, I'm not too confident in my solution. Could someone possibly confirm this as well?

2. Originally Posted by sregg43
I have some math problems that I'm confused on...

I was given these formulas to work with:

...
$A(t) = A_{0}b^{tk}$ <<<< this one is sufficient for your problems
...

If 500 fleas triple in their population every 18 hours:
1) How many fleas will there be in 3 days?
I got 40500, but will someone confirm whether this is right? <<<< correct!

2) How long will it take for there to be 1200 fleas?
I got 14.34 hours, but again, I'm not too confident in my solution. Could someone possibly confirm this as well?
1. I assume that you use the general equation

$A(t)=500 \cdot 3^{k \cdot t}$

with $k = \frac1{18}$ and t measured in hours.

2. You have to solve for t:

$1200 = 500 \cdot 3^{\frac1{18} \cdot t}$

$\frac{12}{5} = 3^{\frac1{18} \cdot t}$

Now use logarithms. BTW I've got the same result.