If the radiological half life of a material is 31 minutes, and you have 77 grams

Question

now, how much did you have 55 minutes ago?

Answer 1

k = 0.0.693/t_1/2
k = 0.693/31 = 0.0224 min^-1

ln(No/N) = kt
No = unknown
N = 77
k = from above.
t = 55 min

Answer 2

look at the intro to nuclear physics (2500U) tutorial notes (tut 6 i think)