12a. 0.417mg
13a. 0.283mg
14. 9.85 x 10^-5 mg
15b. 39.1 days
16. 6.2 days
17. 0.97:1 (wtf how did i even get this)
18a. 317.6 years
18b. 1694
19. 4710 years
20. 6.71 x 10^9 years