[hockey101]

Ok, so I need to know if I'm doing this half-life problem correctly. 

The Question states: 

What is the 1/2 life of Tc-99 if a 200g sample decays to 50g in 639,000 yrs?

My work:

Ao = 200g
An = 50g
t1/2 = ?
t = 639,000 yrs.
n = ?

So.... :

n = ln (200/50)
             .693
n = 2

Then.... :

t1/2 = 639,000/2    =    319,500 yrs.

[UG]

Ok, so I need to know if I'm doing this half-life problem correctly. 

Answer looks good.