The half-life of 234U, uranium-234, is 2.52 105 yr. If 97.7% of the uranium in the original sample is present, what length of time (to the nearest thousand years) has elapsed

Answer 1

Since we have a "half-life" situation I can use a base of 1/2

.977 = (1/2)^(t/2.52x10^5)
t/2.52x10^5 = ln.977/ln.5
t = 2.52x10^5(ln.97/ln.5) = 8459.22 years or 9000 years to the nearest thousand years

for an alternate solution see MathMate's
http://www.jiskha.com/display.cgi?id=1310960571