You'll be left with 6.0% of the initial dose.
So, you know that potassium-42 has a nuclear half-life of 12.4 hours. This means that the initial dose of potassium-42 gets halved every 12.4 hours.
Since the numbers don't allow for a quick calculation, you're going to have to use the nuclear hal-life equation
The first thing you need to do is convert the time given to you from days to hours
This means that you get
To get the percentage of the initial dose that remains after 50.4 hours, divide
Rounded to two sig figs, the answer will be