Exponential function word problems?

I really don't enjoy word problems. Can you give me a hand with the following? Once it's in an equation I'm pretty good..

5. For a biology experiment, the number of cells present is 1000. After 4 hours, the count is estimated to be 256000. What is the "doubling period" of the cells?

* How do you derive an equation from this? I tried and got x= 1/2 as my answer. Am i anywhere close?

6. A certain bacteria strain divides every 0.5 hours. If 500 bacteria are present in a culture, how many will be there

a) After 3 hours?

B) After 6 hours?

C) After n hours?

10. A certain strain of yeast cell doubles every 20 minutes. If there were 350 initially, how many will there be in 3 hours?

Again guys, I could really use the help.

Thanks so much!