Posted by Andrea on .
The radioactive isotope you discovered at 8 a.m weighed 75 mg. You named it "Nerdium." At 3 p.m. it measured only 0.009 grams. What is its halflife?

algebra 
Steve,
using mg as the basis,
9 = 75*(1/2)^7x
x = 0.437 hrs
makes sense, since 9/75 is about 1/8 or 3 halflives 3/7 =0.43