4.375 g of the carbon 11 would be remaining after 80 minutes.
Half life is the time taken for a substance to decay to half of its original value. It is given by:
[tex]N(t)=N_o(\frac{1}{2})^{\frac{t}{t_\frac{1}{2} } } \\\\\\Where\ t_\frac{1}{2}\ is\ half\ life,N_o\ is\ original\ amount,t\ is\ time,N(t)\ is\\amount\ remaining\\\\\\Given\ that\ N_o=70,t_\frac{1}{2}=20,t=80, hence:\\\\N(t)=70(\frac{1}{2} )^\frac{80}{20}=4.375\ g[/tex]
Therefore, 4.375 g of the carbon 11 would be remaining after 80 minutes.
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