F = 9/5C + 32
Subtract 32 to both sides:
F - 32 = 9/5C
Divide 9/5 to both sides or multiply by its reciprocal, 5/9:
5/9(F - 32) = C
Simplify:
C = 5/9F - 160/9
120 grams half life at 45 days would be 60 then half life of 60 in another 45 days (90 total) should be 30 grams
