afew wrote:The half-life of potassium-40 is approx. 1.31 billions years. analysis of some rocks around some dinosaur bones indicated that 94.5% of the original amount of potassium-40 was still present. Estimate the age of the bones of the dinosaur.

One customarily uses the "continously-compounded" formula for

this sort of exercise.. . . . .You are given that, in t = 1.31 (where "t" is measured in "billions of years"), A = (1/2)P. That is:

. . . . .. . . . .Solve the exponential equation for the value of the decay constant "r".

Now that you have the value of "r", you can do the second part of the exercise: finding the age, given an amount. Whatever the initial amount P was, you now have A = 0.945P left. So plug these into the equation, and solve for the time "t".

If you get stuck, please reply showing how far you have gotten. Thank you!