Relax, settle down, forget about the problems for a minute, and think
about "half-life" until you're sure you understand what it means.
"Half life"means the length of time it takes for half of the shaky atoms that
you started with to fall apart. If you start with 100 of them, then 50 of them
die in the first half-life. If you start with 2 million of them, then 1 million of
them die in the first half-life. If you start with 2 radioactive atoms, then
one or the other will decay during the first half-life.
In this problem, you started with 96 grams of radioactive chromium.
Half of it ... 48 grams ... decays away in the first half-life.
What happens after that ? Exactly the same thing.
You start the next half-life with 48 grams of radioactive chromium.
Half of that ... 24 grams ... decays away in the next half-life.
After two half-lifes, you have 1/4 of what you started with.
