A 100-gram sample of an unknown radioactive substance decays to 55% of its initial mass in 7 hours. Note: The mass of the sample is modeled by the formula M(t) = Moekt. (A) Find the growth constant k. ku Your answer must be exact. Write "In" or "e" if necessary. (B) Find the mass of the sample after 18 hours. mass= grams Your answer must be exact. Write "In" or "e" if necessary. (C) Find the time in hours when the sample will have a mass of 28 grams. time= hours Your answer must be exact. Write "In" or "e" if necessary.