If 100 g of Au-198 decays to 6.25 g in 10.8 days, what is the half-life of Au-198?
Half life of 2.7 days
Since Au-198 is 100 g originally and it decays to 6.25 g,
You'd divide 100 g by 2 until you reach 6.25 g:
Since there were four 2s, divide 10.8 days by 4:
100 g of Au-198 would have a half life of 2.7 days.