The half-life of radium is 1,690 years. how long will it take for a 50-gram sample of radium to be reduced to 5 grams? The answer should be 5,614 years.

The formula I'm using is Q(t)=Q_{0}e^{-kt}and I believe I am to use h= ln2/k. Here's what I've done:

5=50e^{(-k)(1,690)}

.1=e^{(-k)(1,690)}

ln .1=(-k)(1,690)

I divide by 1,690 and get -5.917...

I'm not sure where to use the h= ln2/k, I tried putting it in for k, it didn't look right so I erased it all and went back to square one. Any help is greatly appreciated. :]