Answer: [tex]3.125\ g[/tex]
Explanation:
Given
Initial mass [tex]A_o=50\ g[/tex]
half-life is [tex]T_{\frac{1}{2}}=5\ years[/tex]
At any time the left amount is given by
[tex]A=A_o2^{-\dfrac{t}{T_{\frac{1}{2}}}}\\\\\text{Insert the values}\\\\A=50\cdot 2^{-\dfrac{20}{5}}\\\\A=50\cdot 2^{-4}\\\\A=\dfrac{50}{16}\\\\A=3.125\ g[/tex]
