The half-life of a radioactive isotope is 140 days. In how many days does the decay rate of a sample of this isotope decrease to one fourth its initial decay rate? A) 35 days B) 70 days C) 105 days D) 210 days E) 280 day

The volume of a spherical hot air balloon v(r) = 4 3 ? r3 changes as its radius changes. the radius is a function of time given by r(t) = 3t. find the instantaneous rate of change of the volume with respect to t at t = 2.