Final answer:
The present activity of the Cobalt-60 source in millicuries is 0.50 mCi. Determining the time elapsed since the Cobalt-60 source had an activity of 4.00 mCi requires using a decay formula with its half-life, but specific calculations are not provided here.
Step-by-step explanation:
The question involves converting the units of radioactivity from becquerels (Bq) to millicuries (mCi) and calculating the decay time of a Cobalt-60 source using its known half-life. Given the current activity of the Cobalt-60 source is 1.85 × 107 Bq, we need to convert this to mCi using the conversion factor where 1 Ci = 3.70× 1010 Bq. For determining how long ago the source had an activity of 4.00 mCi, we would use the known half-life of Cobalt-60 (5.26 years) and the decay formula.
(a) Using the conversion factor 1 Ci = 3.70 x 1010 Bq, we can find the current activity in mCi. 1 mCi = 1/1000 Ci = 3.70 x 107 Bq. Therefore, the present activity in mCi is (1.85 x 107 Bq) / (3.70 x 107 Bq/mCi) = 0.50 mCi.
(b) To calculate how long ago the Cobalt-60 source had an activity of 4.00 mCi, a decay formula should be used incorporating the half-life of Cobalt-60. However, without providing the decay formula and computational steps, exact calculation of the time elapsed cannot be included in this answer.