A radioactive sample with a half-life of 10 minutes is placed in a container.
a) Complete the pictures below depicting the amount of this sample at the beginning of the experiment ( $t=0$ min) and 30 minutes into the experiment $(t=30 \mathrm{~min})$. Each sphere represents one radioactive nuclide of the sample.
(b) A friend working with 1000 atoms of the same nuclide represents the amount of nuclide at three time intervals in the following manner.
Is his drawing correct? If not, explain where and why it is incorrect.