Answer:
At the 3rd half life there are 13 atoms remaining.
Step-by-step explanation:
Initially we start with a model of potassium-40 atoms which has 50 atoms of it.
We have to find how many atoms would remain at the third half life.
A half life is the time duration with which the number of atoms reduce to half of it's initial number.
At the first half life 50 would reduce to it's half and only 25 would remain .
At the end of the second half life or the starting of the third half life there would be 12.5 atoms remaining. As we cannot express number of atoms as a decimal we round it off to 13.
The number of atoms = 13