The half-life of argon is 10 years, which means that after 10 years, half of the original amount of argon will remain.
Initially, the fossil had 40 grams of argon. After one half-life (10 years), the amount of argon left would be:
\[ \text{Remaining argon} = \frac{40 \text{ g}}{2} = 20 \text{ g} \]
So, after 10 years, the fossil would have 20 grams of argon left.