Need Help for Half Life Problems using Logs

Hi, I need help with solving half life problems using natural log. Or normal log. I learned this before, but totally forgot how to do it right now before the Final Exam, please help. (Doh)

The equation that I got was:

N = No (1/2) ^ t

N is final amount, No is the initial ammount and t is time.

I was given the data:

Time (min) 10 20 30 40

Amount (g) .83 .68 .56 .46

and I need to find the regression equation, write an equation in terms of base e, and use the equation to predict the half life.

How do I do this? I have no clue... (Doh)