You can put this solution on YOUR website!
This same problem came up about a week ago, here is what I submitted then
The level of thorium in a sample decreases by a factor of one-half every 4.2 million years. A meteorite is discovered to have only 7.6% of its original thorium remaining. How old is the meteorite?
The decay formula: Ao*2^(-t/h) = A
Ao = initial amt
A = resulting amt
t = time (in millions of yrs)
h = half-life of the substance (in millions of years)
In this problem: let Ao = 1; A = .076
1*2^(-t/4.2) = .076
ln(2^(-t/4.2)) = ln(.076); find the nat log of both sides
.693 = -2.577; use the log equiv of exponents
Multiply both sides by 4.2
-.693t = -2.577 * 4.2
-.693t = -10.823
t = 15.6 million years old
Check solution on a calc enter 2^(-15.6/4.2) = .076..