Americium-242 has a half-life of 6 hours. If you started with 24 g and you now have 3 g, how much time
has passed?

The time required by Americium-242 to reduce to 3 grams from 24 grams has been 18 hours.

Half-life can be described as the time required by the substance to reduce to half of its initial concentration.

The half-life can be expressed as:

Final concentration = Initial concentration

0.125 =

0.125 can be written as:

0.125 =

Since the bases are same:

3 =

t = 18 hours.

The time required by Americium-242 to reduce to 3 grams from 24 grams has been 18 hours.

For more information about the half-life refer to the link:

link