A radioactive sample has an initial mass of 120g. Five hours later, the mass is 80g. Assuming the mass M(t) (in grams) decays at a rate proportional to the amount present, approximately how many additional hours (beyond the initial 5 hours) will pass before the mass decreases to 50g?
a) 3.8h
b) 5.8h
c) 10.8h
d) 8.1h