Question

The half-life period of the yttrium is 20 min. If there
are 1000 g of yttrium present, how much will be
left after 30 min?

Answer 1

The radioactive decay law says

$$m(t)=m_0\times 2^{-t/t_{1/2}}$$

Hence

$$m(t)=1000\:\rm g\times 2^{-30/20}=354\: g$$