How much of the beginning mass remains after 70 years?

Simply replace t by 70:

$${\mathtt{y}} = {\mathtt{40}}{\mathtt{\,\times\,}}{\left({\frac{{\mathtt{1}}}{{\mathtt{2}}}}\right)}^{\left({\frac{{\mathtt{70}}}{{\mathtt{40}}}}\right)} \Rightarrow {\mathtt{y}} = {\mathtt{11.892\: \!071\: \!150\: \!027\: \!210\: \!7}}$$

y = 11.89 grams to the nearest one-hundredth of a gram

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Wow.. it was that easy?! Thanks so much Alan!