Three half-lives for Nitrogen-13, with a half-life of 10 minutes, amount to a total time of 30 minutes.
The student is asking how long three half-lives are for Nitrogen-13, which has a half-life of 10 minutes.
In the example given, one full half-life passage would result in half the starting mass, leading to 1 g of Nitrogen-13 after 10 minutes.
After two half-lives (20 minutes total), the amount would again halve to 0.5 g.
Then, after three half-lives (30 minutes total), we would expect the amount to halve once more, leaving 0.25 g of Nitrogen-13 from the initial 2.00 g.
This can also be illustrated by the provided example with tritium, where each half-life results in the mass being halved consecutively at set time intervals.