If the half-life of a given substance is 60 days, how long will it take for a 30-gram sample of the substance to decay until there are only 7.5 grams of the radioactive material remaining?

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.

Because a model would make the investigation easier to understand