4. (25 marks)

A sample of uranium-239 (U-239) decays into neptunium-239(Np-239),according to the following standard decay function N(t)=Noe -t,t0 where N(t) is the amount(in mg) of U-239 after t minutes, N0 is the initial amount (in mg) of U-239, is the disintegration constant, t is time elapsed (in minutes)

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

a) Find the disintegration constant for U-239.

(Keep 2 significant figures for your result.)

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

(Keep 4 significant figures for your result.)