Hi, I'm reviewing for a test that will happen this week and I need some help on these problems..

1) A yeast colony is growing exponentially, it starts with 300 individuals and 4 hours later it has 5000 individuals.

A) How long will it take to have 20,000 individuals?

b) What is the doubling time for this colony?

2) A radioactive substance has a half life of 8,000 years, how long does it take to lose 1% of the substance to radioactive decay?

In these problems, I'm just not getting how to figure out the time with the initial amount known and multiplying factor known... I tried doing some calculating but they aren't coming out right

I can do problems like these where the time is given but I have not been taught to find the time with only other info given

Any help would be appreciated