The rate of decay is .005

yO represents the initial amount of radioactive material

If 95% of the sample has decayed then only 5% remains. If we started with 100 grams then we are looking for when only 5 grams are left so

5= 100e^(-005t)

.05 = e^(-.005t) Now take the natural log of both sides which gives us

ln.05 = -.005t

t= ln.05/-(.005)

t=599.146 years