Exponential growth & decay

[summersnow]

A round table alleged to have belonged to King Arthur was found at Winchester Castle. In 1976 carbon dating revealed the amount of radiocarbon in the table to be 91% of the radiocarbon present in living wood. Could the table possibly have belonged to King Arthur? Explain. (carbon 14 has half-life of 5730 yrs).

So far I set Q=.5Q when t=5730 yrs.
.5 = e^5730k
ln(.5) = lne&5730k
ln(.5)/5730 = k

How would I set up the 91% radiocarbon w/ the half-life?

 

[tkhunny]

use your value for k and solve 0.91 = exp(k*t) for t.