Half-life is the duration needed for a compound to decay into half of its initial weight. In this case, the initial weight is 100gram. The calculation for the final weight should be:
final weight= initial weight * (1/2)^(duration/half-life)
final weight= 100 grams * (1/2)^(20days/5 days)
final weight= 100 grams * (1/2)^(4)
final weight= 100 * 1/16= 100/16 gram= 6.25 grams
