The nuclide 55Ca has a half-life of 2.737 years, decaying by electron capture to 55Mn. Find the mean life and the decay constant for this radionuclide. A source of this isotope has an activity of 0.900 mCi. Find the activity remaining after the elapse of 4.2 years from the measurement just given. Express the result in millicuries and becquerels.