If the half-life is 15 hours and there are 18 g, it means that after 15 hours,
18/2 grams would decay, leaving behind 18/2 grams
This means that 9 grams would be left.
After another 15 hours (making a total of 30 hours), 9/2 grams would decay leaving behind 9/2 grams (i.e 4.5 grams.
After another 15 hours (making a total of 45 hours), 4.5/2 grams would decay leaving behind 4.5/2 grams. Hence 2.25 grams are left
After another 15 hours (making a total of 60 hours), 2.25/2 grams would decay leaving behind 2.25/2 grams.
Hence after 60 hours, only 1.125 grams would be left