Think about what a half-life means as a table.
years amount Left
20 1/2
40 1/4
60 1/8
80 1/16
Starting at t = 0, the amount left (or density) = (1/2)^(t/20)
For t =35, the decay factor = .297
Since you need to work backwards, divide by that factor. 6 / .297 = 20.18 mg per sq cm
