Exponential Functions/Logs Questions -- Help!

2 .Assume that the number of viruses present in a sample is modeled by

the exponential function f(t) = 10t, where t is the elapsed time in

minutes.

How would you apply logarithms to determine when the sample will grow

to 5 billion viruses?

4. Maya has deposited $600 in an account that pays 5.64% interest, compounded continuously. How long will it take for her money to double.

I have the following:

A = Pe^rt

A = 600e^(5.64)(t)

1200 = 600e^(5.64)(t)

2 = e^(5.64)(t)

ln(2) = ln(e)^(5.64)(t)

ln(2) = 5.64t

(ln(2))/5.64 = t

Is this correct?

7. A computer is infected with the Sasser virus. Assume that it infects 20 other computers within 5 minutes; and that these PCs and servers each infect 20 more machines within another 5 minutes, etc. How long until 100 million computers are infected?