Question 395072
The half life of thorium-229 is 7340 yrs. 
how long will it take a sampleof this substance to decay to 20% of its original amount? 
:
I think the equation should be:
A = Ao*2^(-t/h)
where
A = amt after t yrs
Ao = initial amt
t = time in yrs
h = half-life of the substance
:
Using the initial amt, Ao = 1, then A = .2
2^(-t/7340) = .2
Using natural logs
{{{-t/7340}}}*ln(2) = ln(.2)
{{{-t/7340}}}= {{{ln(.2)/ln(2)}}}
{{{-t/7340}}}= -2.3219281
t = -7340 * -2.3219281
t = 17,043 yrs
:
:
Check solution on calc:
 enter 2^(-17043/7340), result: .19999 ~ .2, confirms our solution of t = 17,043 yrs