2. An isotope of sodium has a half-life of 15 h. A sample of this isotope has a mass of 2 g.
a.  Find the mass equation that remains after t hours.
b.  Find the mass that remains after 5 hours, to the nearest hundredth.
c.  How long, to the nearest hour, will the sample take to decay to a mass of 0.4 grams?
d.  Find the average rate of change between 15h and 45h.