Question 251614
I think you wrote the problem down wrong.  As this is in the logarithm section and the formula is most likely exponential.  I think the problem should be:
f(t)=100e^rt OR f(t)=100*6^t
I don't think you can find the "doubling time" your originally written equation because t(0) (the sales in 1998) is 0.
.
Email me at justin.sheppard.tech@hotmail.com and I will be happy to assist you further.  Include "algebra" as the subject line incase it gets diverted to my junk email folder.

Thanks,
Justin