Answer
R: 20 seconds after
Work Step by Step
Half-life of a decay is the time it takes for half of the nuclei in a sample of a radioactive element to decay. For a decay with a half-life of 10 seconds, this means that after 10 seconds, half of the initial number of nuclei will have decayed into the daughter nucleus. After another 10 seconds, half of the remaining nuclei will have decayed, and so on.
At t = 0 seconds, there are 1000 A nuclei and no B nuclei. After the first 10 seconds, the number of A nuclei is halved to 500, and the number of B nuclei becomes 500.
After the next 10 seconds, the number of A nuclei is halved again to 250, and the number of B nuclei becomes 750.
So, the number of B nuclei will be 750 after t = 10s + 10s = 20s
