The radioactivity from a sample of Na₂ ¹⁴CO₃ was measured by noting the time required for the count of disintegrations to reach 8000. Five times were recorded: 21.25, 23.46, 20.97, 22.54, and 23.01 min. Background radiation was measured by noting the time required for 500 counts to be registered. Three times were recorded: 5.26, 5.12, and 4.95 min. What is the average level of radioactivity in the Na₂ ¹⁴CO₃ sample, corrected for background radiation, in

(a) Disintegrations per minute;

(b) Microcuries?