Use the decay curve for C$-14$ shown in Figure $1$ to answer questions $6$ and $7.$

Percentage of Carbon$-14$

Figure $1$

$6.$ The remains of two different organisms are analyzed. The first organism has $9\%$ of its original C$-14$ remaining. The second organism has $30\%$ of its original C$-14$ remaining. $$

$($a$)$ What is the approximate age of the remains of the first organism?

$($b$)$ What is the approximate age of the remains of the second organism?

Why is carbon$-14$ rather than carbon$-15$ used for radioactive dating of the remains of organisms?