An exponential decay problem.

A sample of uranium-239 (U-239) decays into neptunium-239 (Np-239) according to the standard decay function:

http://img199.imageshack.us/img199/1849/expfunction.png

After 10 minutes, the sample has decayed to 64% of its initial amount.

a) The value of the disintegration constant, pi, is approximately 0.045/min

b) The half-life of U-239 is 15.5 minutes approximately.

(working on this at the moment)

c) Write the equation that gives the amount of U-239 remaining as a function of time, in terms of its half-life.

d) Suppose the initial sample had a mass of 25mg. How fast is the sample decaying after 15 minutes?