Answer is: 120 days will it take for decay.
Half-life is
the time required for a quantity (in this example number of radioactive nuclei of lincolnium) to
reduce to half its initial value.
Initial mass of sample: 30 grams.
After first half-life (60 days): 50% · 30 g ÷ 100% = 15 g.
After second half-life (120 days): 0.5 · 15 g = 7.5 g.
