Radioactive decay follows an exponential model, where the amount of a nuclide decreases by half after each half-life period. Starting with 6.0g, after one half-life of 8 days, 3.0g remains, and after another 8 days (16 days total), 1.5g remains. To decay from 6.0g to 0.75g, an additional half-life of 8 days is needed, totaling 24 days for 5.25g to decay.