Exponential growth and decay questions (2)

I've been struggling with these two exponential growth and decay problems. :(

In http://hosted2.webwork.rochester.edu...3992170c21.png days, an unknown radioactive substance decay to http://hosted2.webwork.rochester.edu...bd5a5a68d1.png percent of its size.
(a) What is the half life of this substance?

(b) How long will it take for a sample of 100mg to decay to http://hosted2.webwork.rochester.edu...909f940f71.png mg?


Also,

A cell of some bacteria divides into two cells every 10 minutes.The initial population is 4 bacteria.
(a) Find the population after t hours


(b) Find the population after 9 hours.


(c) When will the population reach 36?

For the bacteria one, I tried having the equation look like this:

8=4e^k10