Two radioactive isotopes of iron, ⁵²Fe and ⁵³Fe, are both positron emitters with half-lives of 8.28 hours and 8.51 minutes, respectively. A particular sample of iron initially emits positrons at a rate of 210 counts per minute. What would be the activity of the sample after 30 minutes if the original sample contained the following?

(a) Only ⁵²Fe, 

(b) Only ⁵³Fe, 

(c) A mixture of 50% of each isotope