12a. 0.417mg
13a. 0.283mg
14. 9.85 x 10^-5 mg [hey kanav this is 2 sig figs :3 since they gave you 8.5 mg instead of the 25.0 mg given in the previous problem)
15b. 39.1 days
16. 6.2 days
17. 0.97:1 (wtf how did i even get this) [lololol i think this is correct, i got .99 : 1 but you have to multiply the ratio by 100 because there are decimals..... i think)
18a. 317.6 years
18b. 1694 [2011 - 317.6 = 1693.4 therefore it is 1693 ^^]
19. 4710 years [got 4697 and needed 3 sig figgggzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz so put 4.70 x 10^3]
20. 6.71 x 10^9 years YES.

I hope you get this lolol I don't even know what pastebin is, ugh still have to study apush fml. The rest of the answers which i didn't add comments to matched!